Start FlexiNotifs

* Adds a basic composable that draws the notification placeholder
* Moves the SharedNotificationContainer to be a sibling of the SceneContainer (under SceneWindowRootView)
* Demonstrates the composable setting data on a view model
* Demonstrates the other view model setting data on the NSSLController + AmbientState

Bug: 296118689
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT
Test: flexiglass on + enable flexi_notifs
Change-Id: If04403534a481b0420596aa6e38d8a9c10f2fc26
35 files changed