Added more unit tests for UserManagerService.

This CL adds tests for the following 3 methods:

- boolean isUserVisible(int userId)
- boolean isUserVisibleOnDisplay(int userId, int displayId)
- int getDisplayAssignedToUser(int userId)

It also adds UnitManagerInternalTest and refactors the test infra
so tests for methods whose implementation is the same for UMS / UMI
(i.e., where the method of the former calls the method of the latter
or vice versa) are written just once.

Bug: 244452695
Test: atest FrameworksMockingServicesTests:com.android.server.pm.UserManagerServiceTest UserManagerInternalTest

Change-Id: I95b53935b8fa01edeb64816e2afda57d4852c888