Implement TextNodeRegion

Summary: NodeRegion is only able to describe a rectangular region for touch, which is not good enough for text, where we want to be able to assign different touch ids to individual words (and those can span more than one line and in general have non-rectangular structure). This diff adds TextNodeRegion which inserts additional markers into text Layout to allow individual words to have unique react tags.

Reviewed By: ahmedre

Differential Revision: D2757387

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

@@ -30,6 +30,10 @@ import android.text.Layout;
     mLayout = layout;
   }
 
+  public Layout getLayout() {
+    return mLayout;
+  }
+
   @Override
   public void draw(FlatViewGroup parent, Canvas canvas) {
     float left = getLeft();