[msbuild] fix for JavaSourceJar and test (dotnet#1420)

Context: d46abe8

The test `BindingBuildTest.JavaSourceJar` appears to pass under
`xbuild`, but fails under `msbuild`. This seems be true for macOS
or Windows.

Under further inspection, I noticed the `BuildDocumentation` target
was never running under `msbuild`: it was being skipped due to the
Outputs being up-to-date in comparison to the Inputs. To fix the
skipped target,I was able to add an additional input in
`Xamarin.Android.Bindings.targets`.

The line:

    <Copy
        SourceFiles="@(IntermediateAssembly->'$(IntermediateOutputPath)%(filename).xml')"
        DestinationFiles="@(IntermediateAssembly->'$(OutputPath)%(filename).xml')"
        SkipUnchangedFiles="true" />

Implies that this should also be an Input:

    `@(IntermediateAssembly->'$(IntermediateOutputPath)%(filename).xml')`

Additionally, there was some stuff in the test that seemed incorrect:

  * The return value of `bindingBuilder.Build` should have an
    assertion that it is true
  * For some reason the text from `GetIntermediaryAsText()` was being
    passed to `Path.Combine()`. I think this was a typo that wasn't
    intentional.

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/BindingBuildTest.cs

@@ -441,8 +441,8 @@ public String greet (String name, java.util.Date date)
 				binding.OtherBuildItems.Add (new BuildItem ("JavaSourceJar", "javasourcejartest-sources.jar") {
 					BinaryContent = () => Convert.FromBase64String (sourcesJarBase64)
 				});
-				bindingBuilder.Build (binding);
-				string xml = Path.Combine (bindingBuilder.Output.GetIntermediaryAsText ("docs/Com.Xamarin.Android.Test.Msbuildtest/JavaSourceJarTest.xml"));
+				Assert.IsTrue (bindingBuilder.Build (binding), "binding build should have succeeded");
+				string xml = bindingBuilder.Output.GetIntermediaryAsText ("docs/Com.Xamarin.Android.Test.Msbuildtest/JavaSourceJarTest.xml");
 				Assert.IsTrue (xml.Contains ("<param name=\"name\"> - name to display.</param>"), "missing doc");
 			}
 		}

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Bindings.targets

@@ -530,7 +530,7 @@ Copyright (C) 2012 Xamarin Inc. All rights reserved.
   </Target>
 
   <Target Name="BuildDocumentation"
-	  	Inputs="@(JavaDocIndex)"
+	  	Inputs="@(JavaDocIndex);@(IntermediateAssembly->'$(IntermediateOutputPath)%(filename).xml')"
 	  	Outputs="@(IntermediateAssembly->'$(OutputPath)%(filename).xml')"
 	  	Condition="'@(JavaDocIndex)' != ''">
     <MDoc