UserVisibilityMediator refactoring, step 2.

The previous refactoring introduced a startUser() on
UserVisibilityMediator and changed UserManagerInternal to call both
startUser() and assignUserToDisplay(); this CL "merges" these two
methods into just startUser(), and changed UserController.startUser()
workflow so the logic of whether the user is visible is handled by
UserVisibilityMediator.

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

Bug: 244644281

Change-Id: Ide7b62c1ab3bb119dde7dc2790919ad3c864b3e9