Add full screen user switcher to lock screen

- Add a FULLSCREEN_USER_SWITCHER status bar state;
- Switch to this state when lock screen is shown and fullscreen
  user switcher is enabled;
- Add a build config and a test boolean system setting to enable
  fullscreen switcher;
- Placeholder GridView UI for fullscreen user switcher;

Bug:19908596
Change-Id: Ief1bc987532522245f4172beca19053d34004cee