[PB] Preventing wallpaper become empty after merge transition.

While BackHandler failed to merge second transition and forward the
transition to next handler, exclude the wallpaper from merge into the
second transition, otherwise the wallpaper might become empty because
the wallpaper isn't a target in the second transition.

The wallpaper cannot be reparent back to display because core didn't
prepare that transaction in Transition#buildFinishTransaction.

Also preventing from loop calls in applyFinishOpenTransition:
onTransitionFinished -> Transitions#playTransition ->
applyFinishOpenTransition again, although the second calls will be break
in Transitions#onFinish, but shouldn't let it happen.

Flag: com.android.window.flags.migrate_predictive_back_transition
Bug: 363922376
Test: set default home to 3rd-party launcher, launch app then trigger
back gesture.

Change-Id: I13554424da632f53e765401272e5670b76c71e99