We are not moving the restriction from system to the DO in the end.
clearDeviceOwnerUserRestrictionLocked becomes the permanent solution
for DeviceOwner CTS. Looks like no one setting DISALLOW_ADD_USER
directly in UserManager except DO/PO, and so remove it when DO is
clear
Change-Id: I235bebebd02b5e0d9883eea6dd3a4e49b40fe043
Fix: 33476323
Test: runtest frameworks-services -c com.android.server.devicepolicy.DevicePolicyManagerTest