Add support for updating adjacent separators on row highlight to FlatList

Summary:
A nice bit of polish for apps is to update the separators between list items
when press actives the highlight (things get especially noticeable/ugly when
the separators are not full width and don't adjust). This can be difficult to
coordinate and update efficiently, so we bake the functionality into
`VirtualizedList`.

The approach taken here is a little different from `ListView`. We pass a new
`separators` object with `renderItem` that has easy-to-use callbacks for toggling
the 'highlighted' prop on both adjacent separators - they can be wired up
directly to the `onShow/HideUnderlay` props of `TouchableHighlight` (pit of
success and all that - we want those RN apps to be polished!), but we also
provide a more generic `separators.updateProps` method to set any custom
props. This also passes in `leadingItem` so more custom wiring can be done on
initial render (e.g. linking the highlight state with `Animated`).

This also moves the separator rendering into the `CellRenderer`. I think this might
also fix some subtle measurement bugs with the `onLayout` not capturing the
height of the separator, so that's nice too, but the main reason is to have
an easy place to store the state so we don't have to re-render the entire list
like `ListView` does. Instead we track references to the cells and call update
only on the two we care about so the feedback is instantaneous even with big,
heavy lists.

This diff also messes with a bunch of flow and updates the example to be more
like a standard list.

`SectionList` support is coming in a stacked diff.

**TestPlan**

Video demo:

https://youtu.be/uFE7qGA0mg4

Pretty sure this is backwards compatible....

Reviewed By: thechefchen

Differential Revision: D4833557

fbshipit-source-id: 685af46243ba13246bf280dae8a4223381ce8cea

Examples/UIExplorer/js/MultiColumnExample.js

@@ -97,7 +97,6 @@ class MultiColumnExample extends React.PureComponent {
         </View>
         <SeparatorComponent />
         <FlatList
-          ItemSeparatorComponent={SeparatorComponent}
           ListFooterComponent={FooterComponent}
           ListHeaderComponent={HeaderComponent}
           getItemLayout={this.state.fixedHeight ? this._getItemLayout : undefined}
@@ -115,15 +114,18 @@ class MultiColumnExample extends React.PureComponent {
     );
   }
   _getItemLayout(data: any, index: number): {length: number, offset: number, index: number} {
-    return getItemLayout(data, index);
+    const length = getItemLayout(data, index).length + 2 * (CARD_MARGIN + BORDER_WIDTH);
+    return {length, offset: length * index, index};
   }
   _renderItemComponent = ({item}) => {
     return (
-      <ItemComponent
-        item={item}
-        fixedHeight={this.state.fixedHeight}
-        onPress={this._pressItem}
-      />
+      <View style={styles.card}>
+        <ItemComponent
+          item={item}
+          fixedHeight={this.state.fixedHeight}
+          onPress={this._pressItem}
+        />
+      </View>
     );
   };
   // This is called when items change viewability by scrolling into or out of the viewable area.
@@ -142,7 +144,18 @@ class MultiColumnExample extends React.PureComponent {
   };
 }
 
+const CARD_MARGIN = 4;
+const BORDER_WIDTH = 1;
+
 const styles = StyleSheet.create({
+  card: {
+    margin: CARD_MARGIN,
+    borderRadius: 10,
+    flex: 1,
+    overflow: 'hidden',
+    borderColor: 'lightgray',
+    borderWidth: BORDER_WIDTH,
+  },
   row: {
     flexDirection: 'row',
     alignItems: 'center',