Explicitly close Mojo proxies during destruction of a FlutterView (#2646)

Partial fix for https://github.com/flutter/flutter/issues/2950

sky/shell/platform/android/io/flutter/view/FlutterView.java

@@ -206,6 +206,10 @@ public class FlutterView extends SurfaceView
         getHolder().removeCallback(mSurfaceCallback);
         nativeDetach(mNativePlatformView);
         mNativePlatformView = 0;
+
+        mSkyEngine.close();
+        mDartServiceProvider.close();
+        mFlutterAppMessages.close();
     }
 
     @Override