Fix scrollTo when using RefreshControl on Android

Summary:
The issue is that the ScrollResponder mixin assumes that the native scrollable node is the top level element and gets it using `React.findNodeHandle(this)` but since Android wraps the native `ScrollView` component with the `RefreshControl`, it finds the native `RefreshControl` node instead and the scroll command gets ignored because it doesn't exists.

This adds a hook to ScrollResponder mixin to allow specifying what is the native scrollable node.

The bug can be reproduced using this https://gist.github.com/janicduplessis/871c0b6d3ad0acaacba9 in UIExplorer.

Fixes #5725
Closes https://github.com/facebook/react-native/pull/5736

Reviewed By: svcscm

Differential Revision: D2896125

Pulled By: nicklockwood

fb-gh-sync-id: 0a00a20551421982c5bc519c542774877ba15c9b

Libraries/Components/ScrollView/ScrollView.js

@@ -355,6 +355,10 @@ var ScrollView = React.createClass({
     return this;
   },
 
+  getScrollableNode: function(): any {
+    return React.findNodeHandle(this.refs[SCROLLVIEW]);
+  },
+
   getInnerViewNode: function(): any {
     return React.findNodeHandle(this.refs[INNERVIEW]);
   },