Remove over-aggressive stack movement.

Moving a task to the bottom was also moving its stack to the back.
Since chrome always finishes by calling moveTaskToBack this put
the home stack in front of the application stack and would cause
bizarre window layering effects.

Definitely fixes 10764463.
Maybe fixes 10678010.

Change-Id: Ic202ae4fad44b36a8444255764d2135fccd4743d