[flexiglass] Prevent notifications from flickering on unlock

Keep the alpha 0 when notifications aren't supposed to be showing during
transitions, for AOD->GONE and LOCKSCREEN->GONE with bypass enabled.

Fix: 376676082
Fix: 390995816
Test: manually verify the transitions, including the insecure camera
launch
Flag: com.android.systemui.scene_container

Change-Id: If380c2820db377875af5f0e2bc4021e46c3da5b3