Delegated user visibility changes from UserController to UserVisibilityMediator.

Fixes: 244333150

Test: atest FrameworksMockingServicesTests:com.android.server.pm.UserManagerServiceTest UserVisibilityMediatorMUMDTest UserVisibilityMediatorSUSDTest UserControllerTest # unit
Test: atest MultipleUsersOnMultipleDisplaysTest CtsMultiUserTestCases:android.multiuser.cts.UserManagerTest # CTS
Test: adb logcat -D -b events | egrep 'I um_'

Change-Id: If5b5519f0a9cd32a0f58447eda8b8a3d168489bb