add NullPointerCheck for serviceStateTracker

ServiceStateTracker is not created inside GsmCdmaPhone, thus there is no
SST inside SipPhone. Add a null pointer check inside CarrierActionAgent.
In the long term we should move CarrierActionSignal and
CarrierSignalAgent from Phone to GsmCdmaPhone

Bug: 64104240
Test: Manaul test by making a SIP call
Change-Id: Ia48ff128c9656446058816043cc825a1f60c8a20
(cherry picked from commit 5ddb6ec3)