Add support for ImageLoadEvents to RCTImageView

Summary: This diff implements ImageLoadEvents (ON_LOAD, ON_LOAD_END and ON_LOAD_START) in RCTImageView. ON_ERROR and ON_PROGRESS are easy to implement, too, but these 2 are supposed to carry extra information (error message and progress) but ImageLoadEvent doesn't support a payload yet (I'll add them in a separate patch).

Reviewed By: ahmedre

Differential Revision: D2824772

ReactAndroid/src/main/java/com/facebook/react/flat/InlineImageSpanWithPipeline.java

@@ -99,6 +99,11 @@ import com.facebook.infer.annotation.Assertions;
     Assertions.assumeNotNull(mCallback).invalidate();
   }
 
+  @Override
+  public void onImageLoadEvent(int imageLoadEvent) {
+    // ignore
+  }
+
   @Override
   public void onAttached(FlatViewGroup.InvalidateCallback callback) {
     mCallback = callback;