Fix incorrect handling of undefined call-screening service

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 and unit test
Bug: 189464381
Change-Id: Ic96dfd09e03f3d1b9da8730273e868699b1a9b39
Merged-In: Ib25cb5b0324e95b9cda3e09bf428e11e2b79601e