[flexiglass] Adjust stack alpha during bouncer transition

Make notifs fade out at the same rate as other lockscreen content, while
swiping up to go to the bouncer. Made sure that both LOCKSCREEN->BOUNCER
and BOUNCER->LOCKSCREEN behave correctly.

This emulates the fade(end = 0.2f) behavior from STL.

This also fixes a bug where the stack would flash when pressing a
pulsing notification on AOD. This is because that triggers an
AOD->LOCKSCREEN transition that gets cancelled, followed by a
LOCKSCREEN->BOUNCER transition.

Fix: 438924860
Fix: 438901526
Test: manual with all types of shade
Flag: com.android.systemui.scene_container
Change-Id: Ied4c12708af9acd5da2cb8b4efc72a1a74706806