Implement sticky headers in JS using Native Animated

Summary:
This re-implements sticky headers in JS to make it work on Android.

The only change that was needed was to expose a way to attach a an animated value to an event manually since we can't use the Animated wrapper and `Animated.event` to do it for us because this is implemented directly in the `ScrollView` component. Simply exposed `attachNativeEvent` that takes a ref, event name and event object mapping. This is what is used by `Animated.event`.

TODO:
- Need to check why momentum scrolling isn't triggering scroll events properly on Android.
- Remove native iOS implementation
- cleanup / fix flow

**Test plan**
Test the example list in UIExplorer, test the ListViewPaging example.
Closes facebook/react-native#11315

Differential Revision: D4450278

Pulled By: sahrens

fbshipit-source-id: fec8da2cffce9807d74f8e518ebdefeb6a708667

UIExplorer/js/UIExplorerExampleList.js

@@ -201,6 +201,7 @@ const styles = StyleSheet.create({
     backgroundColor: '#eeeeee',
   },
   sectionHeader: {
+    backgroundColor: '#eeeeee',
     padding: 5,
     fontWeight: '500',
     fontSize: 11,