Expose a meta-data value to allow dialer ringing

This CL exposes a new meta-data, IN_CALL_SERVICE_RINGING. If this is set
to true then ringing is played by the dialer instead of Telecom.

This CL also adds a new silenceRinger() API to InCallService. This is
needed to implement ringer silence on volume key down.

BUG: 22857261
Change-Id: I498538282eddbb727104f5b879f25adbef4e6cf6