Add getUserLogoutability

This new hidden API can tell whether if the user can logout.
The new API to actually logout user will be added in follow-up CL.

Bug: 350045389
Bug: 380122753
Flag: android.multiuser.logout_user_api
Test: m
Test: atest FrameworksMockingServicesTests:com.android.server.pm.UserManagerServiceTest

Change-Id: Ia150c423a5a50e1e71aecb6e3299121e7b7580f4