Added UserVisibilityListener logic to UserVisibilityMediator.

This CL only adds logic on UserVisibilityMediator so it can call
the UserVisibilityListener callbacks upon users starting or stopping,
but it doesn't change UserController to use it yet - that will be done
in a separate CL. In fact, there are still some corner-case scenarios
there the logic is not working (for example, when the system user is
switched out, it's not sending a visibility change event for its
profiles)

Bug: 244333150
Test: atest UserVisibilityMediatorMUMDTest \
            UserVisibilityMediatorSUSDTest

Change-Id: Id10462768b905c5e72b5fa9fb33af048dadd2ad0