Add log out to system user in UserLogoutInteractor

Sign out button to keyguard's status bar should log out to
system user only - introduce isLogoutToSystemUserEnabled state flow
and logOutToSystemUser fun for that purpose.

Test: atest UserLogoutInteractorTest
Flag: EXEMPT interactor only change
Bug: 392052370
Change-Id: Ib5e6c6e168c1e4ffce7650941de63ff915815f52