Transition lockscreen alpha to 0f immediately when going from alt bouncer -> primary bouncer

Previously, when lockscreen is either occluded by screensaver or an app, opening alt bouncer from the app and then going to primary bouncer causes the lockscreen UI to flash for one frame.

This fixes that issue.

Fixes: 406362444
Fixes: 406363894
Test: verified manually,
 1. Launch camera over lockscreen
 2. tap on a button that causes alt bouncer to popup
 3. swipe up to go to bouncer
 4. lockscreen UI should not flash in-between.

Flag: com.android.systemui.bouncer_ui_revamp
Change-Id: If75375d3899371dc388b11d1079cbf44a8ef7331