Workaround to force show TaskFragment while back gesture animating.

Unlike Task and ActivityRecord, the TaskFragment won't show it's
surface during animating, and the surface can be hide after close
transition in Transitions#setupStartState.

Bug: 271599041
Test: verify the TaskFragment can show during back gesture animation.
Change-Id: I9a3d56d3e6c0cc9f735d82f99fd1fc04c329d690