Reapply "Add smooth dimming transition for swapping in flexible split"

This is a re-land attempt at ag/33204333 with test failures addressed.

Bug: 349828130
Flag: com.android.wm.shell.enable_flexible_two_app_split
Test: Dim layer transitions nicely when swapping. Doesn't affect onscreen > onscreen case. No change when flag is off.
Change-Id: I0ae6d7b3e9ff1acd4bd5a45075a8f11b5c2900b4