[flexiglass] Instantly switch to Gone when unlocking from Bouncer

This fixes the bug where the lockscreen scene would show behind the
bouncer as the bouncer overlay was being hidden due to an unlock.

Fix: 432552235
Test: manually verified that the lockscreen isn't visible as the bouncer
overlay is animating away after a successful unlock. To make sure the
bug was more visible, I dismisses all notifications on the lockscreen
before testing as the large clock makes the bug more obvious.
Flag: com.android.systemui.scene_container

Change-Id: Ic8b9e7beaf4b7a47d8b0f89b4c39f5d7e271302e