Update contrast listener in systemui on user switch

In case of user switch, make sure to unregister the listener for the
previous user and register a new one. Do this via the existing
UserTracker.Callback.

Flag: android.app.fix_contrast_and_force_invert_state_for_multi_user
Test: manual
Bug: 362682063
Change-Id: I07c5e9445131f1300904f1430dc747079bb3f13c