Keep wallpaper in prepare back transition.

Revert part of ag/29115929, it would be safe as long as the wallpaper
surface won't merged into next transition.

Bug: 365500016
Flag: com.android.window.flags.migrate_predictive_back_transition
Test: atest BackAnimationControllerTest
Change-Id: I51232a95a585c1b931a68f456d9f208a6c50cb71