Fix ScrollViewStickyHeader to push up header above it

Summary:
When there are multiple sticky headers, ScrollViewStickyHeader should push up the header above it when it gets to the top.

This behavior was accidentally changed in D21948830 (fa5d3fb) when this component was fixed to work in Fabric.

This diff added a new variable `_shouldRecreateTranslateY`, which determines whether the `translateY` value should be recreated on render. `_shouldRecreateTranslateY` was not being set to true during `setNextHeaderY`, so the next header's Y value was never accounted for at render.

Changelog:
[General][Fixed] Fix ScrollViewStickyHeader to push up header above it

Reviewed By: lunaleaps

Differential Revision: D27277829

fbshipit-source-id: 83c9aacd454be178649bf8d060d1a5c750f4060f

Libraries/Components/ScrollView/ScrollViewStickyHeader.js

@@ -64,6 +64,7 @@ class ScrollViewStickyHeader extends React.Component<Props, State> {
   _debounceTimeout: number = Platform.OS === 'android' ? 15 : 64;
 
   setNextHeaderY(y: number) {
+    this._shouldRecreateTranslateY = true;
     this.setState({nextHeaderLayoutY: y});
   }