Add support for show layout bounds.

Summary: Supports show layout bounds either by override within FlatViewGroup, or if show layout bounds is set in settings.  Currently requires app restart to disable.

Reviewed By: ahmedre

Differential Revision: D3553669

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

@@ -32,4 +32,9 @@ import android.graphics.Canvas;
       parent.drawNextChild(canvas);
     }
   }
+
+  @Override
+  public void debugDraw(FlatViewGroup parent, Canvas canvas) {
+    parent.debugDrawNextChild(canvas);
+  }
 }