support sticky headers

Summary:
This adds support for both automagical sticky section headers in
`SectionList` as well as the more free-form `stickyHeaderIndices` on
`FlatList` or `VirtualizedList`.

The basic concept is to take the initial `stickySectionHeaders` and remap them
to the indices corresponding to the mounted subset in the render window. The
main trick here is that the currently stuck header might itself be outside of
the render window, so we need to search the gap to see if that's the case and
render it (with spacers above and below it instead of one big spacer).

In the `SectionList` we simply pre-compute the sticky headers at the same time
as when we scan the sections to determine the flattened length and pass those
to `VirtualizedList`.

This also requires some updates to `ScrollView` to work in the churny
environment of `VirtualizedList`. We propogate the keys on the children to the
animated wrappers so that as items are removed and the indices of the
remaining items change, react can keep proper track of them. We also fix the
scroll back case where new headers are rendered from the top down and aren't
updated with the `setNextLayoutY` callback because the `onLayout` call for the
next header happened before it was mounted. This is done by just tracking all
the layout values in a map and providing them to the sticky components at
render time. This might also improve perf a little by property configuring the
animations syncronously instead of waiting for the `onLayout` callback. We
also need to protect against stale onLayout callbacks and other fun stuff.

== Test Plan ==

https://www.facebook.com/groups/react.native.community/permalink/940332509435661/

Scroll a lot with and without debug mode on. Make sure spinner
still spins and there are no crashes (lots of crashes during development due
to the animated configuration being non-monotonic if anything stale values get
through). Also made sure that tapping a row to change it's height would
properly update the animation configurations so the collision point would
still be correct.

Reviewed By: yungsters

Differential Revision: D4695065

fbshipit-source-id: 855c4e31c8f8b450d32150dbdb2e07f1a9f9f98e

Examples/UIExplorer/js/SectionListExample.js

@@ -79,6 +79,7 @@ class SectionListExample extends React.PureComponent {
 
   state = {
     data: genItemData(1000),
+    debug: false,
     filterText: '',
     logViewable: false,
     virtualized: true,
@@ -96,6 +97,16 @@ class SectionListExample extends React.PureComponent {
       filterRegex.test(item.text) || filterRegex.test(item.title)
     );
     const filteredData = this.state.data.filter(filter);
+    const filteredSectionData = [];
+    let startIndex = 0;
+    const endIndex = filteredData.length - 1;
+    for (let ii = 10; ii <= endIndex + 10; ii += 10) {
+      filteredSectionData.push({
+        key: `${filteredData[startIndex].key} - ${filteredData[Math.min(ii - 1, endIndex)].key}`,
+        data: filteredData.slice(startIndex, ii),
+      });
+      startIndex = ii;
+    }
     return (
       <UIExplorerPage
         noSpacer={true}
@@ -111,6 +122,7 @@ class SectionListExample extends React.PureComponent {
           <View style={styles.optionSection}>
             {renderSmallSwitchOption(this, 'virtualized')}
             {renderSmallSwitchOption(this, 'logViewable')}
+            {renderSmallSwitchOption(this, 'debug')}
             <Spindicator value={this._scrollPos} />
           </View>
         </View>
@@ -124,6 +136,7 @@ class SectionListExample extends React.PureComponent {
           ItemSeparatorComponent={() =>
             <CustomSeparatorComponent text="ITEM SEPARATOR" />
           }
+          debug={this.state.debug}
           enableVirtualization={this.state.virtualized}
           onRefresh={() => alert('onRefresh: nothing to refresh :P')}
           onScroll={this._scrollSinkY}
@@ -139,7 +152,7 @@ class SectionListExample extends React.PureComponent {
               {noImage: true, title: '1st item', text: 'Section s2', key: '0'},
               {noImage: true, title: '2nd item', text: 'Section s2', key: '1'},
             ]},
-            {key: 'Filtered Items', data: filteredData},
+            ...filteredSectionData,
           ]}
           viewabilityConfig={VIEWABILITY_CONFIG}
         />