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

RNTester/js/ScrollViewExample.js

@@ -131,7 +131,7 @@ if (Platform.OS === 'ios') {
   exports.examples.push({
     title: '<ScrollView> smooth bi-directional content loading\n',
     description:
-      'The `maintainPositionAtOrBeyondIndex` prop allows insertions to either end of the content ' +
+      'The `maintainVisibleContentPosition` prop allows insertions to either end of the content ' +
       'without causing the visible content to jump. Re-ordering is not supported.',
     render: function() {
       let itemCount = 6;
@@ -146,7 +146,10 @@ if (Platform.OS === 'ios') {
             <View>
               <ScrollView
                 automaticallyAdjustContentInsets={false}
-                maintainPositionAtOrBeyondIndex={1}
+                maintainVisibleContentPosition={{
+                  minIndexForVisible: 1,
+                  autoscrollToTopThreshold: 10,
+                }}
                 style={styles.scrollView}>
                 <ActivityIndicator style={{height: 40}} />
                 {this.state.items.map(item =>
@@ -156,9 +159,12 @@ if (Platform.OS === 'ios') {
               <ScrollView
                 horizontal={true}
                 automaticallyAdjustContentInsets={false}
-                maintainPositionAtOrBeyondIndex={1}
+                maintainVisibleContentPosition={{
+                  minIndexForVisible: 1,
+                  autoscrollToTopThreshold: 10,
+                }}
                 style={[styles.scrollView, styles.horizontalScrollView]}>
-                <ActivityIndicator style={{height: 40}} />
+                <ActivityIndicator style={{width: 40}} />
                 {this.state.items.map(item =>
                   React.cloneElement(item, {key: item.props.msg, style: null}),
                 )}