Bring fullscreen backdrop frame back

We might need to bring the legacy split-screen back. This CL restores
the logic of the fullscreen backdrop frame.

Bug: 161810301
Test: Drag the split-screen divider after setting
      ActivityTaskManager#setSplitScreenResizing to true
Change-Id: Ifdc8b7695595e81e381b3311cf0d266f9af4be23