Formalize ConnectionService#onCreateXComplete hidden APIs.

These were added a long time ago for the Telephony stack so that it could
know when Telecom has finished adding a new connection or conference.
The original design of ConnectionService#onCreateConnectionComplete and
ConnectionService#onCreateConferenceComplete assumes that the developer
returns their instance of Connection or Conference, respectively, from
those APIs and the platform then adds those to Telecom.

The original API design there has a small timing issue in that if you
want to signal on the Connection/Conference, you need to wait until it has
been added to Telecom first.  In practice this is not often a problem,
well, until it is.  We've had a few 3p VOIP apps running into similar
issues so making these public gives them the same benefit we've seen
in the Telephony stack.

Test: Added new CTS test coverage for these APIs.
Fixes: 317391843
Change-Id: I46b00e468873bc7f880f6a4bd2416adfa0a4c161