Create `ObservableState` to replace some Consumers

This is used in NotificationPlacehodlerRepository instead of a consumer pattern to ensure our types reflect the fact that these are states.

Flag: com.android.systemui.scene_container
Bug: 387961133
Test: atest ObservableStateTest
Test: atest QuickSettingsShadeOverlayContentViewModelTest
Test: atest NotificationStackAppearanceInteractorTest
Change-Id: Ia63b7884f6e935007228391b4d57b22e909bbdc2