Support indicating call direction on existing connections.

This is important for ensuring the original call direction for existing
connections added from Conference Event Packages matches the call direction
of the original calls merged into the conference.

Also moved a utility function into ConferenceParticipant from
ConferenceParticipantConnection to make it generically usable inside
Telephony.

Test: Run all unit tests.
Test: Manual testing using VoLTE conference calls with mix of MO and MT
calls; verify call logging is appropriate.
Bug: 134471046

Change-Id: Iab09397b811782ab0f876aac02070e3447d81f09