Cleanup hub disabled flow by consolidating logic

The isCommunalAvailable flow now controls whether the view gets added to
the shade at all. If the flow is false, the view is removed completely.

This makes the flow the source-of-truth for whether communal hub is
actually available or not.

Test: atest CommunalTutorialInteractorTest
Test: atest CommunalAppWidgetHostStartableTest
Test: atest CommunalInteractorTest
Test: atest CommunalViewModelTest
Bug: 319331389
Flag: ACONFIG com.android.systemui.communal_hub STAGING
Change-Id: I7e0d272eee0579494ad240fcc3c7543dfd3546c2