Add further Connection-side APIs for RTT (part 2)

Add methods and callbacks to facilitate local and remote RTT initiation
and termination in the middle of a call. Adds @hide Connection-side APIs
to communicate with the ConnectionService, as well as plumbing for
RemoteConnections.

Test: manual, through telecom testapps
Merged-In: Ia80604b7dff8586ff222dbccdbe55e91aab02178
Change-Id: Ia80604b7dff8586ff222dbccdbe55e91aab02178