UserSwitcherController impl uses UserInteractor

Based on a flag, UserSwitcherControllerImpl will use UserInteractor,
GuestUserInteractor, and KeyguardInteractor to implement its interface.

Once this is all cleaned up, we can remove the old implementation and
even update users of UserSwitcherControllerImpl to directly use the
interactors instead.

Bug: 246631653
Test: Unit tests added for newly exposed functionality. Existing tests
still pass. Manually verified the full-screen user switcher and the
dropdown in the bouncer.

Change-Id: Iff7c06f8c0e2fdd8adaab436ff895e9d892d2f20