RTT bugfixes, part 5

* Add a new API to allow Telecom to inform ConnectionServices when the
RTT text stream changes
* No longer set the RTT property from ConnectionService. Client apps
should be setting properties themselves.
* Add Dialer-side RTT property in order to remove the dependence on
checking the RTT streams, which have a complex lifecycle

Bug: 72951201
Bug: 72648661
Test: manual, with real RTT calls and Dialer's SimulatorConnection, also
cts
Change-Id: Ic4c7d883d2dc6baf8e8c0eaa4df58d7de8762b9e