Dispatch JS calls to JS thread from c++ bridge

Summary: Instead of dispatching calls to the JS thread in Java, do it in the C++ bridge. This moves us closer to the cxx bridge and will allow us to dispatch to the correct web worker in C++ instead of in Java

Reviewed By: mhorowitz

Differential Revision: D2954115

fb-gh-sync-id: 7e7d4eff2c72601b8b4416f1ccd8d2985aebd755
shipit-source-id: 7e7d4eff2c72601b8b4416f1ccd8d2985aebd755

ReactAndroid/src/androidTest/java/com/facebook/react/testing/ReactIntegrationTestCase.java

@@ -70,14 +70,17 @@ public abstract class ReactIntegrationTestCase extends AndroidTestCase {
       mReactContext = null;
       mInstance = null;
 
+      final SimpleSettableFuture<Void> semaphore = new SimpleSettableFuture<>();
       UiThreadUtil.runOnUiThread(new Runnable() {
         @Override
         public void run() {
           if (contextToDestroy != null) {
             contextToDestroy.destroy();
           }
+          semaphore.set(null);
         }
       });
+      semaphore.getOrThrow();
     }
   }