Added more unit tests for UserManagerInternal.

This CL adds tests for the following 2 methods:

- void assignUserToDisplay(int userId)
- void unassignUserFromDisplay(int displayId)

It also makes some minor changes on those methods to throw exceptions
on invalid usage.

Bug: 244452695
Bug: 244644281

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

Change-Id: Ieb17e7e8a43f7d697f98a45d1b0660d999ca28ec