Adding ITelecommService definition for Telecomm.

Until telecomm code moves into a system service, we need a way for
other apps to call into it for call-related functionality.

Initial implementation only has silenceRinger.
This is to be implemented by the telecomm code and used by
TelephonyManager (until we have a TelecommManager).

Change-Id: I9180797451dcb2e9029b20bed47f5d5cb8cddb9f
(cherry picked from commit b895606acf18dbb050c9a32106b3c9c2e5111829)