Creating connections for conference event package participants.

- Add "addExistingConnection" method to connection service which provides
a way for a connection service to notify telecom of a pre-existing
connection (connections are normally created through telecom).
- Modify TelephonyConferenceController to retrieve its state from a
multiparty connection in the conference (in the case of IMS calls, this
would be the ImsCall that manages the conference) instead of just taking
the first one.

Bug: 18057361
Change-Id: I26993aec54ecb0ce90ae6983fd3eed9d8d0a5773