[flexiglass] Update NSSL alpha during alt->prim bouncer transition

Flag: com.android.systemui.scene_container
Test: manual
1. Have biometric auth setup
2. Go to lockscreen
3. Tap a notification, alt bouncer shows
4. Tap on screen, primary bouncer shows
Observe: notifications do not appear over primary bouncer
Fixes: 379847927

Change-Id: Ib9f8f284c8c6197909ea10143b9f3de7d5ca89ed