[CS] 2/N: Define WallpaperRepository.wallpaperSupportsAmbientMode.

This will replace CentralSurfacesImpl.mWallpaperChangedReceiver. This CL
moves ScrimController to the new repo; other controllers will be moved
in future CLs.

Reasoning around NoopWallpaperRepository: Previously,
CentralSurfacesImpl was the only class calling
ScrimController.setWallpaperSupportsAmbientMode. CentralSurfacesImpl is
only included in AOSP SysUI and SysUIGo. So, the new WallpaperRepository
should also only be enabled for AOSP SysUI and SysUIGo. However,
ScrimController is included in *more* than just those two variants.
NoopWallpaperRepository should be used for those variants to ensure that
they can still use ScrimController and also still *not* pay attention to
wallpaper changes.

Bug: 277762009
Bug: 277764509
Test: manual: verify via logging that WallpaperRepository re-calcuates
supportsAmbientMode at the same pace as CentralSurfacesImpl with the
same values. Specifically verify the value is re-calcuated whenever the
wallpaper is changed or the user is switched.
Test: manual: switch users *without first switching the wallpaper* and
verify via logging that the repository re-fetches supportsAmbientMode.
(This specifically tests that the `wallpaperChanged` flow uses `.onStart
{ emit(Unit) }`, which was missing in the first iteration of this CL.)

Test: atest WallpaperRepositoryImplTest ScrimControllerTest
Change-Id: Icb69f842c52c19d89857218d07077ad90d83c1e6
Merged-In: Icb69f842c52c19d89857218d07077ad90d83c1e6