Skip scheduleStopOfBgUser tests if visibleBg

Some devices support visible background full users. These users are
expected to run in the background and serve a valuable purpose. They
should not be scheduled for stopping just because they're in the
background. The UserController code already handles this, but the test
currently doesn't exempt such devices, so we update the test here.

Bug: 360920454
Test: atest FrameworksServicesTests:com.android.server.am.UserControllerTest
Flag: android.multiuser.schedule_stop_of_background_user
Change-Id: I8688caa87c880b5edbe863e99f8417b6c0bfb9e7