Flexible 2-app split: Dimming (part 2)

This CL adds dim layers to the recents transition, so that they correctly fade in and out when going from splitscreen to recents and back.

Remaining issues:
- A small dark flash when the app initially settles in the 90:10 ratio. Deferring this because I suspect it will be fixed by an upcoming app surface change.

Bug: 349828130
Flag: com.android.wm.shell.enable_flexible_two_app_split
Test: Visually correct when a 90:10 pair goes to recents and back
Change-Id: I053b587f6fbeabf27e3dfbc15fe81f17768db78f