Disable user switching in SysUI for desktop

This is a temporary solution, until proper handling of desktop
switching restrictions are implemented at the framework level.

For now use config_userSwitchingMustGoThroughLoginScreen to
override visibility of user switching UI.

Test: tested manually
Test: atest UserRepositoryImplTest
Flag: EXEMPT Desktop-only change
Bug:375394047
Bug:378068979
Change-Id: Ife7b8ffff8352d6421ef7c385bf30f83fad932bc