[flexiglass] Implement Shade -> Lockscreen scene transition

Adds a notification scrim in Lockscreen scene that fades out after the user lifts their finger during/after the Shade -> Lockscreen scene transition, and uses the alpha of said scrim to also drive NSSL alpha during this interaction.

Bug: 347763135
Test: manually verified transition looks correct
Flag: com.android.systemui.scene_container
Change-Id: I709b0e1fc188b3cd0b6554837ac757af049c816f