Flexible 2-app split: Dimming (part 1)

Implements dimming effects for apps going offscreen. This does not handle the case where the user swipe up to go to Overview in Launcher; that will be fixed in part 2.

Bug: 349828130
Flag: com.android.wm.shell.enable_flexible_two_app_split
Test: Visually correct when dragging divider in flex and non-flex split
Change-Id: I8f3ad4c9a33f688383f537247becdedf800b67f6