Fixing when the full-screen switcher appears.

Now, full screen user switcher always shows up on reboot.

The switcher is dismissed after "switch" to another user
is detected.
Switching should lead to authentication directly

Bug: 74445625
Test: Repeated boot and switch on Mojave, with single and many users.
Change-Id: Ia1645246276d9eb65288c01c8c320c82a796d442