Simplifies show/dismiss logic for the full screen user switcher.

Switcher should only be shown when Cancel is clicked on the bouncer. Otherwise,
the entire keyguard should get dimissed, leading to the bouncer or user's home activity.

Fixes the issue where a keyguard is not dismissed after turning the screen off and back on.

Change-Id: Idd2c8f9f5bf6b490f287b5c5777f39e69437970f
Fixes: 112358232
Test: Manual tests on both multi-user and single-user builds. Repeated boots, switcher and screen off/ons for users with and without pin/password set.