Transitions - Fix scrim flash as bouncer goes away

When tapping a notification and being prompted by the bouncer, the
animation should start from the notification and not from the
bouncer. Fade the bouncer away quickly to show the animation from the
keyguard. This was most noticeable in user switcher mode.

Fixes: 271823107
Test: atest PrimaryBouncerToGoneTransitionViewModelTest
Change-Id: I97e5883e962a3315f534b4d41d6a16b7ae107889