Fixes a race condition when merge animation transition

When merge a TO_FRONT transition to the recents animation,
onTasksAppeared will finish the recents animation, so there can be a
race that the open transition for recents animation might be finish
before the merge transition, which leave the merge transition become
a standalone transition.
To make the sequence correct, register a transition commit callback
to know the merge transition has done in Shell, then goes to finish
the recents animation.

Bug: 235433368
Test: enable shell transitin, quick switch back/forward and verify no
flicker.

Change-Id: I39fc898a8922f900e1e1808f92a471749664e194