[Flexiglass] Base NSSL alpha off bouncer transition, and INVISIBLE when occluded

NSSL should correctly fade out as the bouncer overlay fades in, and become View.INVISIBLE once the bouncer is fully faded in. This should also fix inline reply functionality.

Bug: 398890669
Bug: 390503464
Test: manually verified NSSL alpha change during Shade -> Bouncer transition
Test: manually verified inline reply works
Test: new unit test forthcoming
Flag: com.android.systemui.scene_container
Change-Id: Ice93016bce30abce5497b07412cacc039cfd7d1c