Fixes dragging divider up in splitscreen puts the docked stack behind

Fixes the regression introduced in ag/1721556 where the docked app would
be placed on top of all the other apps when it should be placed behind
everything.

Change-Id: I63fc3bcfd0ef93f6ac8509a4268b628d9b0eb46b
Fixes: 34697887
Test: manual - split screen and drag up to dismiss