Summary:
* Added snapToOffsets prop to ScrollView. Allows snapping at arbitrary points.
* Fixed pagingEnabled not being overridden by snapToInterval on iOS.
* Fixed Android *requiring* pagingEnabled to be defined alongside snapToInterval.
* Added support for decelerationRate on Android.
* Fixed snapping implementation. It was not calculating end position correctly at all (velocity is not a linear offset).
* Resolves https://github.com/facebook/react-native/issues/20155
* Added support for new content being added during scroll (mirrors existing functionality in vertical ScrollView).
* Added support for snapToInterval.
* Resolves https://github.com/facebook/react-native/issues/19552
Reviewed By: yungsters
Differential Revision: D9405703
fbshipit-source-id: b3c367b8079e6810794b0165dfdbcff4abff2eda
fd744dd56c