Clear DISALLOW_ADD_USER user restriction when dpm remove-active-admin is called from shell

- Make the CTS hermetic and not adding new restriction after CTS test
- DeviceOwner can't clear this user restriction in CTS, as it's set by ManagedProvisioning
- It can be only clear when testOnly DPC becomes deviceOwner, and being removed by remove-active-admin in shell

Bug:31856203
Test: build successfully

Change-Id: I75b91629ef09c54e9dbe7253df6a52894a938e83