[flexiglass] Snap to Scenes.Gone after boot

As soon as the SceneContainer is initialized, the view model checks
if the lock screen should be shown as initial scene and if not will
initiate a switch to Scenes.Gone. This is making sure that the lock
screen will no longer be shown after boot when no screen lock is
set. Setting the screen lock to "Swipe to enter" will make the
lockscreen appear on boot as expected.

Test: manually tested boot, show/hide shade, open/close keyguard,
switch users, create new user and go through SuW with flexiglass on
and off using different screen lock options
Flag: com.android.systemui.scene_container
Bug: 412925277. 360372242

Change-Id: Id70f101a059bfdd26f112d532af8fdbfcde2ea51