Add scrim transitions to/from glanceable hub

The glanceable hub is accessible from keyguard, but since it's
underneath all of the scrims, so that notifications/bouncer can show
over it, it's also dimmed by the lockscreen's back scrim which applies
a slight dimming to the lockscreen wallpaper.

This change adds a new ScrimState for the hub and implements a
transition to/from it as well as adjusting existing logic so that
opening the notification shade and bouncer on top of it work as
expected.

Bug: 315203484
Fixed: 315203484
Test: atest CentralSurfacesImplTest ScrimControllerTest
Flag: ACONFIG com.android.systemui.communal_hub DEVELOPMENT
Change-Id: I577e352eda47af4a03ffdff2aaa788f6f853238c