rename and extend new maintain visible content position feature

Summary:
Builds off of https://github.com/facebook/react-native/commit/cae7179c9459f12b1cb5e1a1d998a9dc45f927dc

- Make the prop a dictionary for more configuration options
- Rename `maintainPositionAtOrBeyondIndex` -> `maintainVisibleContentPosition` + `minIndexForVisible`
- Add autoscroll threshold feature

Given the async native of RN JS and background layout, there is no way to trigger the scrollTo from JS without risking a delay, so we add the feature in native code.

== Test Plan ==
ScrollViewExample:
https://youtu.be/pmY8pxC9PRs

Reviewed By: shergin

Differential Revision: D6729160

fbshipit-source-id: 70f9bae460ce84567857a4f696da78ce9b3b834c