Fix termination of local ringback on SRVCC.

The previous code fix was doing it on handover from VOWIFI to LTE, which
is not correct.
This change ensures it happens only on actual SRVCC complete.

Bug: 157020264
Bug: 156863251
Test: Added new unit test to verify ringback ceases on SRVCC.
Change-Id: I69c33e3de1e51e616637ff5c9412429b66221983