[flexiglass] AlternateBouncer support

The AlternateBouncer shows as its owe distinct UI
above the Lockscreen and Shade scenes when the
scene container flag is enabled.

Add AlternateBouncer over the shade logic to drive
the transition to the AlternateBouncer showing over
the Shade/Notifications/Quick Settings.

Ensure AlternateBouncer can show over occluding
apps by updating SceneContainerStartable#hydrateVisibility
to make SceneContainer visible when altBouncer is visible.

Flag: com.android.systemui.scene_container
Bug: 353955910
Test: with UDFPS or SFPS enrolled, expand the shade on the
lockscreen and tap on a notification that requires auth;
observe the AlternateBouncer shows and user can auth
from this screen. User can also tap the scrim for
the primary bouncer.
Test: atest AlternateBouncerViewModelTest

Change-Id: Ia9b60737d55c4fd0f670773173eedd12066e546d