[Java.Interop] Remove JniRuntime.JniValueManager.GetObject()

Context: https://bugzilla.xamarin.com/show_bug.cgi?id=25443
Fixes: dotnet/java-interop#2
Fixes: dotnet/java-interop#3

The problem with JniRuntime.JniValueManager.GetObject() is that it was
constrained to only return IJavaPeerable instances.  This constraint
in turn necessitated the introduction of JniMarshal.GetValue<T>()
(commit 1e14a61) so that "anything" could be stored within a
JavaObjectArray<T>, proxying values if required.  This in turn led to
a dichotomy: should JniRuntime.GetObject<T>() be used, or should
JniMarshal.GetValue<T>() be used?

This dichotomy was partially resolved in commit 77a6bf8, which
removed JniMarshal.GetValue<T>() and added a
JniRuntime.JniValueManager.GetValue<T>() method, but that just
introduced a different dichotomy: should JniValueManager.GetObject()
be used or JniValueManager.GetValue()?

Simplify this mess: kill JniValueManager.GetObject() (and all
overloads), and instead rely on JniValueManager.GetValue().

Additionally, add JniValueManager.CreateValue() overloads (Issue #3)
and fix JniValueMarshaler<T>.CreateGenericValue() logic to *not*
lookup registered values, as per commit 77a6bf8:

> clean[] up JniValueMarshaler.Create*Value() methods to always
> create values when able, simplifying the logic of
> JniValueMarshaler implementations.

Cleaning up these semantics is necessary so that
JniValueManager.CreateValue() can be meaningful.  The only time that
JniValueManager.CreateValue() won't create a *new* value is if the
value is a proxy'd instance, at which point we're not going to muck
with it and any resulting bugs are Not Our Problem™.

The primary change of killing JniValueManager.GetObject() required a
few secondary changes:

  * JniValueManager.RegisterObject<T>() can no longer throw a
    NotSupportedException when attempting to create an alias, as
    (1) the NotSupportedException is a semantic change from
    Xamarin.Android that we can't accept -- aliases are a fact of
    life, and will be required for .JavaCast<T> (Issue #10) -- and
    (2) the check was breaking unit tests.
    We now generate a warning to the GREF log when aliases are
    created, similar in spirit to what Xamarin.Android does.

  * ProxyValueMarshaler.CreateGenericValue() now falls back to
    JniValueManager.CreateObjectWrapper() instead of returning `null`.
    This was needed so that ValueManager.GetValue<Exception>(...)
    would work.

  * For a "unified type system"-like experience,
    JniValueManager.CreateObjectWrapper() now maps certain types to
    corresponding types, e.g. object is mapped to JavaObject, and
    Exception is mapped to JavaException.  This is likewise to support
    ValueManager.GetValue<Exception>(...) around a Java instance.

src/Java.Interop.Dynamic/Java.Interop.Dynamic/JavaConstructorInfo.cs

@@ -42,7 +42,7 @@ public override unsafe object Invoke (IJavaPeerable self, JniArgumentValue* argu
 		{
 			if (self == null) {
 				var h   = members.InstanceMethods.StartCreateInstance (JniSignature, typeof (JavaInstanceProxy), arguments);
-				self    = JniEnvironment.Runtime.ValueManager.GetObject<JavaInstanceProxy> (ref h, JniObjectReferenceOptions.CopyAndDispose);
+				self    = JniEnvironment.Runtime.ValueManager.GetValue<JavaInstanceProxy> (ref h, JniObjectReferenceOptions.CopyAndDispose);
 			}
 			members.InstanceMethods.FinishCreateInstance (JniSignature, self, arguments);
 			return new DynamicJavaInstance (self);

src/Java.Interop.Dynamic/Java.Interop.Dynamic/JavaMemberInfo.cs

@@ -22,7 +22,7 @@ protected virtual void Dispose (bool disposing)
 
 		protected static object ToReturnValue (ref JniObjectReference handle, string signature, int n)
 		{
-			var instance    = JniEnvironment.Runtime.ValueManager.GetObject (ref handle, JniObjectReferenceOptions.CopyAndDispose);
+			var instance    = JniEnvironment.Runtime.ValueManager.GetValue<IJavaPeerable> (ref handle, JniObjectReferenceOptions.CopyAndDispose);
 			switch (signature [n]) {
 			case 'L':
 				return new DynamicJavaInstance (instance);

src/Java.Interop.Export/Java.Interop/ExportedMemberBuilder.cs

@@ -328,7 +328,7 @@ static Expression GetThis (Expression vm, Type targetType, Expression context)
 		{
 			return Expression.Call (
 					Expression.Property (vm, "ValueManager"),
-					"GetObject",
+					"GetValue",
 					new[]{targetType},
 					context);
 		}

src/Java.Interop.Export/Tests/Java.Interop/ExportedMemberBuilderTest.cs

@@ -178,7 +178,7 @@ public void CreateMarshalFromJniMethodExpression_InstanceAction ()
 		ExportTest __this;
 
 		__jvm = JniEnvironment.Runtime;
-		__this = __jvm.ValueManager.GetObject<ExportTest>(__context);
+		__this = __jvm.ValueManager.GetValue<ExportTest>(__context);
 		__this.InstanceAction();
 	}
 	catch (Exception __e)
@@ -304,7 +304,7 @@ public void CreateMarshalFromJniMethodExpression_FuncInt64 ()
 		long __mret;
 
 		__jvm = JniEnvironment.Runtime;
-		__this = __jvm.ValueManager.GetObject<ExportTest>(__context);
+		__this = __jvm.ValueManager.GetValue<ExportTest>(__context);
 		__mret = __this.FuncInt64();
 		__jret = __mret;
 		return __jret;
@@ -343,7 +343,7 @@ public void CreateMarshalFromJniMethodExpression_FuncIJavaObject ()
 		JavaObject __mret;
 
 		__jvm = JniEnvironment.Runtime;
-		__this = __jvm.ValueManager.GetObject<ExportTest>(__context);
+		__this = __jvm.ValueManager.GetValue<ExportTest>(__context);
 		__mret = __this.FuncIJavaObject();
 		__jret = References.NewReturnToJniRef(__mret);
 		return __jret;

src/Java.Interop/Java.Interop/JavaArray.cs

@@ -129,12 +129,6 @@ internal static Exception CreateMarshalNotSupportedException (Type sourceType, T
 		internal static IList<T> CreateValue<TArray> (ref JniObjectReference reference, JniObjectReferenceOptions transfer, Type targetType, ArrayCreator<TArray> creator)
 			where TArray : JavaArray<T>
 		{
-			var value = JniEnvironment.Runtime.ValueManager.PeekObject (reference);
-			var array = value as TArray;
-			if (array != null) {
-				JniObjectReference.Dispose (ref reference, transfer);
-				return array.ToTargetType (targetType, dispose: false);
-			}
 			return creator (ref reference, transfer)
 				.ToTargetType (targetType, dispose: true);
 		}

src/Java.Interop/Java.Interop/JavaPeerableExtensions.cs

@@ -9,18 +9,5 @@ public static string GetJniTypeName (this IJavaPeerable self)
 			JniPeerMembers.AssertSelf (self);
 			return JniEnvironment.Types.GetJniTypeNameFromInstance (self.PeerReference);
 		}
-
-		internal static object GetValue (ref JniObjectReference handle, JniObjectReferenceOptions transfer, Type targetType)
-		{
-			return JniEnvironment.Runtime.ValueManager.GetObject (ref handle, transfer, targetType);
-		}
-
-		internal static JniObjectReference CreateLocalRef (object value)
-		{
-			var o = value as IJavaPeerable;
-			if (o == null || !o.PeerReference.IsValid)
-				return new JniObjectReference ();
-			return o.PeerReference.NewLocalRef ();
-		}
 	}
 }

src/Java.Interop/Java.Interop/JavaProxyObject.cs

@@ -69,20 +69,21 @@ public static JavaProxyObject GetProxy (object value)
 
 		static bool _Equals (IntPtr jnienv, IntPtr n_self, IntPtr n_value)
 		{
-			var self    = JniEnvironment.Runtime.ValueManager.GetObject<JavaProxyObject> (n_self);
-			var value   = JniEnvironment.Runtime.ValueManager.GetObject (n_value);
+			var self    = (JavaProxyObject) JniEnvironment.Runtime.ValueManager.PeekObject (new JniObjectReference (n_self));
+			var r_value = new JniObjectReference (n_value);
+			var value   = JniEnvironment.Runtime.ValueManager.GetValue (ref r_value, JniObjectReferenceOptions.Copy);
 			return self.Equals (value);
 		}
 
 		static int _GetHashCode (IntPtr jnienv, IntPtr n_self)
 		{
-			var self = JniEnvironment.Runtime.ValueManager.GetObject<JavaProxyObject> (n_self);
+			var self = (JavaProxyObject) JniEnvironment.Runtime.ValueManager.PeekObject (new JniObjectReference (n_self));
 			return self.GetHashCode ();
 		}
 
 		static IntPtr _ToString (IntPtr jnienv, IntPtr n_self)
 		{
-			var self    = JniEnvironment.Runtime.ValueManager.GetObject<JavaProxyObject> (n_self);
+			var self    = (JavaProxyObject) JniEnvironment.Runtime.ValueManager.PeekObject (new JniObjectReference (n_self));
 			var s       = self.ToString ();
 			var r       = JniEnvironment.Strings.NewString (s);
 			try {