propagate getScrollResponder, getScrollableNode

Summary:
so users can call `setNativeProps` and do more compositing.

**Test Plan:**

Added this to `onPress` of `SectionListExample` and `FlatListExample`:

    this._listRef.getNode().getScrollResponder().setNativeProps({scrollEnabled: false});

and saw scroll get disabled. Note the call to `getNode` because we are using the `Animated.createComponent` wrapper.

Reviewed By: achen1, bvaughn

Differential Revision: D4821711

fbshipit-source-id: 8d1f3dd7ccc646524f154721c5c7036620d57132

Libraries/Lists/MetroListView.js

@@ -67,6 +67,9 @@ class MetroListView extends React.Component {
       this.props.horizontal ? {x: offset, animated} : {y: offset, animated}
     );
   }
+  getListRef() {
+    return this._listRef;
+  }
   static defaultProps: DefaultProps = {
     keyExtractor: (item, index) => item.key || index,
     renderScrollComponent: (props: Props) => {