Improve predictive custom cross activity back animation

This changes the behaviour of custom cross-activity back animations. Until now, app-provided custom xml animations were seeked from 0 to 100% for the predictive back animation. With this CL this changes such that the system takes control over the precommit phase whereas the app-provided xml spec is applied to the post-commit animation:
1. The closing activity is scaled during pre-commit (exactly the same as in the default cross-activity-back-animation)
2. A scrim is added on top of the opening activity
3. The closing activity plays custom animation during post commit
4. The opening activity seeks custom animation to max 20% during pre-commit
5. The opening activity plays the remaining % of the custom animation during post-commit

Bug: 339440390
Flag: com.android.window.flags.predictive_back_system_anims
Test: atest CustomCrossActivityBackAnimationTest
Test: Manual, i.e. testing xml  specified cross activity back animations manually in a test app
Change-Id: Iefa1ce9a0169d0cab16deab196e4afa464b44b4e