Summary:
I have an issue when combining `PullToRefreshViewAndroid` and `ViewPagerAndroid`.
`ViewPagerAndroid` will not able to scroll that gesture handler is being taken by `PullToRefreshViewAndroid`
One solution is to disable `PullToRefreshViewAndroid` if `ViewPagerAndroid` is scrolling (i.e. not idle).
[Reference solution here](http://stackoverflow.com/a/29946734/2590265)
So here need to expose the `onPageScrollStateChanged` event.
Some code referenced from DrawerLayoutAndroid, especially the `VIEWPAGER_PAGE_SCROLL_STATES` array.
Please feel free give me comments.
Thanks.
Closes https://github.com/facebook/react-native/pull/5026
Reviewed By: svcscm
Differential Revision: D2830623
Pulled By: andreicoman11
fb-gh-sync-id: c2a6920c6f4c7daab0115f13864db83b93b31abf
8de86a0d65