Don't move stack with task for moveTaskToFront.

Earlier CL, ag/321609, was overly aggressive. It should only have kept
the stack from moving when moveTaskToStack() was called. Instead the
change was applied to moveTaskToFront() as well. In order to remain
consistent with earlier behavior of moveTaskToFront() the stack
containing the task must move to the front as well. This movement
launches the activity at the top of the task. Without the stack
movement the activity never gets launched causing bug 9498455.

Fixes bug 9498455.

Change-Id: I2cb9119c23a226f7d211851df9e49f10f702b3ef