[Java.Interop] Additional marshaler lookup (#389)

Context: #388

Before going to use the proxy object marshaler, try to check the
implemented interfaces of `type` for a custom marshaler.

It can be used in xamarin-android to marshal `IJavaObject` based
objects, which do not implement `IJavaPeerable`.

src/Java.Interop/Java.Interop/JniRuntime.JniValueManager.cs

@@ -527,6 +527,20 @@ public JniValueMarshaler GetValueMarshaler (Type type)
 				if (typeof (IJavaPeerable).GetTypeInfo ().IsAssignableFrom (info)) {
 					return JavaPeerableValueMarshaler.Instance;
 				}
+
+				JniValueMarshalerAttribute ifaceAttribute = null;
+				foreach (var iface in info.ImplementedInterfaces) {
+					marshalerAttr = iface.GetTypeInfo ().GetCustomAttribute<JniValueMarshalerAttribute> ();
+					if (marshalerAttr != null) {
+						if (ifaceAttribute != null)
+							throw new NotSupportedException ($"There is more than one interface with custom marshaler for type {type}.");
+
+						ifaceAttribute = marshalerAttr;
+					}
+				}
+				if (ifaceAttribute != null)
+					return (JniValueMarshaler) Activator.CreateInstance (ifaceAttribute.MarshalerType);
+
 				return GetValueMarshalerCore (type);
 			}