UserVisibilityMediator refactoring, step 4.

Refactors unit tests so the test methods test full scenarios (like
starting a user in foreground and checking its visibility), rather
than individual methods per se.

Such change will make it easier to add tests for new logic (like
callbacks) - it will require adding new assertions for the existing
use cases, rather than dozen of new tests to test the logic in all
use cases.

Test: atest UserVisibilityMediatorMUMDTest UserVisibilityMediatorSUSDTest

Bug: 244644281

Change-Id: I7f09d10ea9579ca7a493462f09750505f1ae6d5b