Initial implementation of HUB<->OCCLUDED transitions.

This allows us to go directly from HUB->OCCLUDED and back, with a custom
fade animation instead of the default fade+translate animation.

Known issues:
1. Keyguard wallpaper occasionally flickers before hub becomes visible.
   Occluded activity is getting removed before hub is visible.
2. Hub fade isn't smooth
3. Expanding activities from widgets while the dream is active is
   broken.
4. Occasional notification flicker when exiting an activity to go back
   to the hub.

Bug: 332591536
Test: atest KeyguardTransitionScenariosTest
Test: atest CommunalSceneStartableTest
Flag: ACONFIG com.android.systemui.communal_hub TEAMFOOD
Change-Id: I5db502a46127a68d709656d2678af76a6dbd12b3