Fix flicker in cross-activity-back-animation

Bug: 347861186
Flag: com.android.window.flags.predictive_back_system_anims
Test: Manual, i.e. verifying that flicker is not reproducible with the CL applied
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:50f9622210e11c7463d9f7a2e95b6940c7b6c33c)
Merged-In: I5464afbffab182e96acae104cf0471bbaf0a056b
Change-Id: I5464afbffab182e96acae104cf0471bbaf0a056b