Add a config flag to disallow removal of last full admin user on HSUM

On HSUM devices, the new flag can disallow the removal of the last
full admin user.
The headless system user is flagged as admin user, and we ignore that
since it's not a full user.

On non-HSUM devices, the last full admin user is the system user.  It
cannot be removed anyway.

This is part of the work to allow to not have main user.

Bug: 391407557
Bug: 394970894
Flag: android.multiuser.disallow_removing_last_admin_user
Test: atest FrameworksMockingServicesTests:UserManagerServiceMockedTest
FCRS_CODE : man7rtia78kiqs

Change-Id: If74e7e049563d0db7ad1db4a05eacae26ffdb7ab