Fix occasional ring volume reset

When PHONE_INCOMING_RING message accidentaly arrives to CallNotifier
immediately after the user answers the call, the call state is still
RINGING instead of OFFHOOK, so ring plays again, causing audio
driver to switch from just entered IN_CALL mode back to RINGING mode,
which can lead to completely mute phone call - no side of call can be
heard.

Above also affects increasing ring feature.

Prevent this scenario by calling silenceRinger() instead of stopRing()
at the beginning of answerCall(), so not only the current ring is
stopped, but also the possible next ring prevented.

Change-Id: I358ffd93f4233ab1a957af18ec40edc0fef04014