Push wallpapers on the back during transitions

Fix: 350617801
Test: Manual, verify wallpaper stays on the back when opening app pair
Test: Manual, unlock device with separate lock/home  wallpapers
Flag: com.android.window.flags.ensure_wallpaper_in_transitions
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:7bc428e7762f59e521ae4e8c8365cba02e7ff82b)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:26a0d430edfbb330e41f6e9741433e1212c88b37)
Merged-In: Ie6bd4d18bfe67ca27b791a666d5b0bff6199773c
Change-Id: Ie6bd4d18bfe67ca27b791a666d5b0bff6199773c