Change the activity reparent to animate at TaskFragment level.

Before, we are animating at activity level, but that wouldn't work after
we set the TaskFragment window crop. Now, we animate the change
transition on the TaskFragment level instead.

Bug: 196173550
Test: atest WmTests:AppTransitionTests
      #testActivityRecordReparentToTaskFragment
Change-Id: Ie777dc8bf438300f59f8a2565cd999c6a29561d8