Ask back navigation target to start rendering asap after gesture finish

After gesture finish, use launch behind to ask app start rendering
before entire close transition finish, the snapshot will be removed once
the opening target report drawn.

Flag: ACONFIG com.android.window.flags.predictive_back_system_anim
Flag: ACONFIG com.android.window.flags.activity_snapshot_by_default
Bug: 328664843
Test: verify snapshot can be removed much faster.
Change-Id: Ifa4ea469c0f882ec51b6d1920d02234afa3e3431