[flexiglass] Integrates scrim state.

- New startable that hydrates scrim state
- Old code paths to set scrim state from CentralSurfacesImpl turned off
- Code to figure out the right ScrimState is an almost intact copy-paste
  from the current code in CentralSurfacesImpl. A couple of differences
  regarding the elimination of callbacks and side-effects (I figure we
  can add those later if we need them)

Bug: 343298208
Flag: com.android.systemui.scene_container
Test: manually verified that the attached bug isn't reproducible anymore
and that the wallpaper is visible when the unfolded/unlocked device is
closed shut
Test: submitting this CL early to ublock others working on Flexiglass;
working on unit tests in followup CL

Change-Id: Iceb6c7441831ab56f3a049d0ec4044e0a29fe262