Add support for RTT calls (part 1)

Adds logic for handling RTT call initiation and communication with the
InCallService and the ConnectionService and includes changes to the
testapps that exercise this functionality. This change handles RTT
initiation from the beginning of a call.

Change-Id: I3d713b662a100b2e0ad817b92005f044bcc60c62
Test: manual, through testapps