Ensure sticky-header is added when calling obtainLithoViewChildren

Summary: When unmounting content with child LithoViews, we collect those LithoViews via the obtainLithoViewChildren API. SectionsRecyclerView was already doing this, but was not adding the sticky-header view which is a LithoView as well. This fix ensures the sticky header is added as well.

Reviewed By: astreet

Differential Revision: D31728554

fbshipit-source-id: ec0321d0a72687f35d8be2ad198d8a7cb4f4d3b7

litho-it/src/test/java/com/facebook/litho/MountStateIncrementalMountWithTransitionsTest.java

@@ -104,7 +104,8 @@ public void incrementalMount_componentOffScreen_mountIfAnimating() {
 
     sectionsRecyclerView.obtainLithoViewChildren(lithoViews);
 
-    final LithoView animatingLithoView = lithoViews.get(1);
+    // grab the 2nd litho-view in the list (0 = sticky header, 1 = 1st litho view)
+    final LithoView animatingLithoView = lithoViews.get(2);
     animatingLithoView.onAttachedToWindowForTest();
 
     mStateCaller.update();

litho-widget/src/main/java/com/facebook/litho/widget/SectionsRecyclerView.java

@@ -215,6 +215,7 @@ public void setOnTouchListener(OnTouchListener listener) {
 
   @Override
   public void obtainLithoViewChildren(List<LithoView> lithoViews) {
+    lithoViews.add(mStickyHeader);
     for (int i = 0, size = mRecyclerView.getChildCount(); i < size; i++) {
       final View child = mRecyclerView.getChildAt(i);
       if (child instanceof LithoView) {