Make sure the lockscreen is not visible behind after Alternate bouncer -> primary bouncer

Bug: 370555003
Test: verified manually,
 1. tap on a notification on the lockcscreen
 2. alternate bouncer will show up as a scrim over lockscreen
 3. swipe up or tap on the scrim
 4. bouncer should show up, and lockscreen should not bo visible behind the bouncer.
Test: unit tests pending, tracked here: b/???
Flag: com.android.systemui.bouncer_ui_revamp
Change-Id: I3a4e8a76f96dd88dc49230136cb22e336d6cee19