Replace onConferenceChanged with onConnectionAdded.

This change is motivated by a desire to be able to adjust the
phone capabilities of a conference depending on its connections.

Turns out Connection is not a good place to do this, because when
the conference is set, this code is only fired if the connection
service already contains the conference. Conference makes more
sense to do this anyways, at the end of the day.

+ Add onConnectionAdded function to Conference.java.
- Remove onConferenceChanged function from Connection.java.

Bug: 18241753
Change-Id: Ib9212a8483a7a7b542618697399bbab8b2701b04