Clean up open/close state for DragLayout.

Fixes: 33105843

-Remove onClose() from DragCallback and introduce CloseCallback.
-Call executePendingTransactions() when popping fragment backstack.
-Rename "onStartDragging()" to "onStartDraggingOpen()" and only call
it eponymously

Change-Id: I4fc30788a7fe3fa017ba051f48941163b3e04079