Trebuchet : Fix overscroll overlay issue

When overscrolling in the app drawer, certain effect/page combinations
cause the pages to be overlayed.

Reproduction :
- Have 2 pages in the app drawer
- Set scroll effect carousel,rotate up/down
- Overscroll one of the pages and let it settle
- Should see first and second page stacked together

Change-Id: Iaa2b06970e2f5d3dda51ce5bf2259523738f7779