Fix scrollView state contentOffset not being in sync with scrollView contentOffset

Summary:
Changelog: [Internal]

# Problem
`scrollView.state.contentOffset` was not in sync with actual `contentOffset` in case `contentOffset` is changed programatically.

# Solution
Add a flag `_isUserTriggeredScrolling` that indicates whether the current scroll is triggered by user or not. In case it isn't, update state.

Reviewed By: shergin

Differential Revision: D20098161

fbshipit-source-id: 021d916e7a45a24095a47bb8f84d1102226b672a