add oss-based fb activity and host classes

Reviewed By: andreicoman11

Differential Revision: D3655436

fbshipit-source-id: 3ca6535cd819171867c79f5e9801d9013676a80f

ReactAndroid/src/main/java/com/facebook/react/ReactNativeHost.java

@@ -90,6 +90,10 @@ public abstract class ReactNativeHost {
     return null;
   }
 
+  protected final Application getApplication() {
+    return mApplication;
+  }
+
   /**
    * Get the {@link UIImplementationProvider} to use. Override this method if you want to use a
    * custom UI implementation.