Apply transfer starting window transition if no other task change.

Transfer starting window usually happen when open/close top activity,
and most of time there could be another open/close task transition
animation is playing. We can apply this reparent transition directly
instead of merging it to current transition animation, so we don't
have to overwrite the merge transition everywhere.

Bug: 211045248
Bug: 211844337
Test: Use trampoline activityA to start another activityB, monitor the
starting window should always on top of the task.
Test: Adjust the time of firstWindowDrawn from activityA to monitor
the transfer transition always smooth.

Change-Id: Ib0523c9f512e4f573d5fa7235cab36e68bfda4d8