Minor changes for MUMD (Multiple Users on Multiple Displays)

- Updated some javadoc / comments
- Removed unnecessary test superclass
- Changed some logs

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

Fixes: 239982558
Fixes: 244644281

Change-Id: Ia5e5a0e50f4b6edce5fc2c9f09cdb492b56cf13e