Supports fullscreen user switcher launched from QS, and enabled by
config_enableFullscreenUserSwitcher. Allows adding guests, with
support for adding new users coming shortly. First round of work on
this feature.
Bug: 217365397
Test: atest UserSwitcherControllerTest
Change-Id: Ib81f44c9b950830a7d89b95cbaef53b7228ed5a4