Moar UserManagerTest improvements:

- Use assertThrows() to test exceptions
- Add test for regular user creation
- Use current user on testCreateUser_disallowAddUser()
- Remove restrictions check from testAddUser()

Test: atest FrameworksServicesTests:com.android.server.pm.UserManagerTest#testGetUserCreationTime

Fixes: 146435179
Fixes: 146501766

Change-Id: I188205e8ea8b63fde536a5f0a215dc11414f59c8