Spencer Ahrens 65184ec6b0 rename and extend new maintain visible content position feature
Summary:
Builds off of cae7179c94

- 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
2018-01-18 14:01:50 -08:00
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