Incorrect handling of undefined call screening

If registration of call-screening service fails (by default it is not
available), the disconnection of said service is still attempted
throwing an error due to an attempt to unregister a receiver that
doesn't exist.

Test: manual testing
Bug: 189464381

Change-Id: Ib25cb5b0324e95b9cda3e09bf428e11e2b79601e