[Flexiglass] Convert Bouncer scene into an overlay

Tasklist:
- All references to Scenes.Bouncer translated to use Overlays.Bouncer
- KTF virtual edges now take ContentKeys
- SceneContainerStartable hides Bouncer overlay after unlock status changes
- NotificationScrollViewModel sets NSSL expansion fraction to 0 when on Notification scene with Bouncer overlay
- Added to ObservableTransitionState API to permit checking if we are idle on an overlay

Bug: 390503464
Test: existing tests updated
Flag: com.android.systemui.scene_container
Change-Id: I44c2b84ea9cb3b5af338abcb4f8533ebbf58c043