[flexiglass] Hydrate ScrimController screen state from ScrimStartable

ScrimStartable wasn't doing this and, instead, the logic was relying on
logic in CentralSurfacesImpl.

This CL effectively moves the logic from CSI to ScrimStartable.

Bug: 416709420
Test: unit tests passing
Test: manually verified that turning the screen on and off doesn't seem
to be doing anything new or special when flexiglass is enabled
Flag: com.android.systemui.scene_container

Change-Id: Id1d1e0ba714f7fc60acc50efdc12bea329c650dc