Initial version of UserVisibilityMediator.

Currently, deciding if user is visible (or visible in a specific
display) or not is handled by UserManagerService directly, and its logic
depends on whether the device supports concurrent multiple users on
multiple displays.

Moving that logic to a separate class will make it easier to support
different modes (for example, passenger-only systems or sharing the
same display among multiple users).

This CL introduces such class, although it's a "pure" refactor that
just moved code (and tests) around "as is"; further improvements
(like breaking its dependency from UserManagerService) will me made
on follow-up CLs.

Test: atest FrameworksMockingServicesTests:com.android.server.pm.UserManagerServiceTest UserVisibilityMediatorMUMDTest UserVisibilityMediatorSUSDTest
Test: atest CtsMultiUserTestCases:android.multiuser.cts.MultipleUsersOnMultipleDisplaysTest
Test: adb shell dumpsys user --visibility-mediator

Bug: 244644281
Change-Id: I4b28e0854c2a14ccebbfe72b4060e1372438324b