Add lockscreen alpha transitions within KTF in Flexiglass

1. Copy alpha flow from KeyguardRootViewModel, and remove flows using
   deprecated KeyguardState
2. Add alpha transition to all elements in lockscreen

Bug: 433166550
Test: manual test transitioning between dozing and lockscreen, camera
and lockscreen, screen saver and lockscreen; observe elements on
lockscreen have alpha transitions.
Flag: com.android.systemui.scene_container

Change-Id: I1eb5d25b1dd37eccc205c68edff03cab59b84ce8