Don't call startRinging twice

Past changes with in-band ringing led to Telecom calling startRinging()
twice, once when the call starts ringing and once when BT audio
connects. Fix this behavior so we don't see ringtone stutters.

Fixes: 159648605
Test: unit tests, manual
Change-Id: I402bb20c39449a7e150aeb1fe8dec7bfd1daa3a4