Fix cancel transition finished before animation finish.

When receive CLOSE_PREPARE transntion, wait for post animation finish
before report transition finish.

Also fix some issues:
- Do not reparent wallpaper to animation leashes, wallpaper doesn't
have to participant animation.
- Lower down the leash of transition root. The root leash should at
the same level of the opening target, but since it is create after
animation, it will gain higher level in surface flinger. Since all
animation still play with animator, move root leash to lower level
so scrim can occludes it.

Flag: com.android.window.flags.migrate_predictive_back_transition
Bug: 363927522
Test: atest BackAnimationControllerTest

Change-Id: I21c4c274ab639bba2be186c85c01a1d7eecddc0d