Apply DISALLOW_ADD_USER to user0 in COPE

DISALLOW_ADD_USER is already applied for the parent user (i.e. personal
profile), but we should also set this on user0 on HSUM devices when boot
to user0 is enabled to avoid being able to add users from the login
screen.

Bug: 398819336
Test: manual
Flag: EXEMPT bugfix & protected by config_hsumBootStrategy
Change-Id: I57adbaf0c362378bb40c6e8aa64ea9bdb3992155