[Java.Interop] Dispose, *then* Remove

[jonpryor]

Fixes: https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1193891

A bug was found in Xamarin.Android 11.1.0.2, candidate for
Visual Studio 16.8 Preview 3:

1. Create a new "Shell Forms App"
2. Build & Run the app
3. Tap the "Browse" button.

Expected results: it works!

Actual results: A rather abrupt crash:

Unhandled Exception:
System.NotSupportedException: Unable to activate instance of type Xamarin.Forms.Platform.Android.PageRenderer from native handle 0x69 (key_handle 0x33efaa1). ---> System.MissingMethodException: No constructor found for Xamarin.Forms.Platform.Android.PageRenderer::.ctor(System.IntPtr, Android.Runtime.JniHandleOwnership) ---> Java.Interop.JavaLocationException: Exception of type 'Java.Interop.JavaLocationException' was thrown.
   --- End of inner exception stack trace ---
  at Java.Interop.TypeManager.CreateProxy (System.Type type, System.IntPtr handle, Android.Runtime.JniHandleOwnership transfer)
  at Java.Interop.TypeManager.CreateInstance (System.IntPtr handle, Android.Runtime.JniHandleOwnership transfer, System.Type targetType)
   --- End of inner exception stack trace ---
  at (wrapper dynamic-method) Android.Runtime.DynamicMethodNameCounter.64(intptr,intptr)
  at (wrapper native-to-managed) Android.Runtime.DynamicMethodNameCounter.64(intptr,intptr)

The crash was caused by commit 007b35b, which changed
JniRuntime.JniValueManager.DisposePeer() semantics from:

value.Disposed ();
RemovePeer (value);

and reversed this to:

RemovePeer (value);
value.Disposed ();

(Why? Unmentioned in 007b35b -- doh! -- but as @jonpryor is the one
who did this, "it seemed like a good idea at the time!")

(Very clearly, this wasn't a good idea.)

The crash was caused by commit 007b35b interacting with Xamarin.Forms'
VisualElementPackager.Dispose(bool), which can call back into
Java code, potentially causing this to need to be returned.
When RemovePeer() is done first, as was done in 007b35b, then
there may be no available instance to return, causing the
NotSupportedException to be thrown.

The new JavaObjectTest.DisposeAccessesThis() test and associated
Java.InteropTests.GetThis type & Java peer trigger this scenario.
Without the fix, DisposeAccessesThis() fails with:

System.NotSupportedException : Could not find an appropriate constructable wrapper type for Java type 'com/xamarin/interop/GetThis', targetType='Java.InteropTests.GetThis'.
  at Java.Interop.JniRuntime+JniValueManager.CreatePeer (Java.Interop.JniObjectReference& reference, Java.Interop.JniObjectReferenceOptions transfer, System.Type targetType)
  at Java.Interop.JavaPeerableValueMarshaler.CreateGenericValue (Java.Interop.JniObjectReference& reference, Java.Interop.JniObjectReferenceOptions options, System.Type targetType)
  at Java.Interop.JniRuntime+JniValueManager.GetValue[T] (Java.Interop.JniObjectReference& reference, Java.Interop.JniObjectReferenceOptions options, System.Type targetType)
  at Java.InteropTests.GetThis.get_This ()
  at Java.InteropTests.GetThis.Dispose (System.Boolean disposing)
  at Java.Interop.JavaObject.Java.Interop.IJavaPeerable.Disposed ()
  at Java.Interop.JniRuntime+JniValueManager.DisposePeer (Java.Interop.IJavaPeerable value)
  at Java.Interop.JavaObject.Dispose ()

demonstrating how RemovePeer(value) must not happen before
value.Disposed().