[RESTRICT AUTOMERGE]Animate back gesture with focusd window activity

Correct the animation target, top most activity can be finishing so
shouldn't be choose as animation target.

Bug: 245467978
Test: Start two activities in a task.
      Finish top activity, before the exit animation done, drive another
      back gesture.
      Verify the target should not point to the top activity.

Change-Id: I4d9d61bd566cafbeef90916cdf40a6fc9c8e13d7