Fade background when keyguard user switcher open

 - Bring back KeyguardUserSwitcherScrim for when the keyguard user
   switcher is opened

 - Remove callback for keyguard user switcher state changes

 - Do not try to change visibility of clock and notifications when user
   switcher is open

Test: Switch between primary user and guest repeatedly
Bug: 179930842
Change-Id: I3f3f10e8fa88c10b023bb74ca5b9d6f623b86516