Change Connections to allow setting conferenceable with conferences.

- Added IConferenceable interface used so that connections and conferences
can both be considered candidates as "conferenceable" with a connection.
- Fixed ConnectionService#conference to support cases where either call 1
or call 2 is a conference and the other is a connection.  Previously did
not support cases where call 2 is a conference.

Bug: 18200934
Change-Id: I32a8dd30a154d6280f2ae89fd147817235998465