Fix issue where GSM conference is held in makeRoomForOutgoingCall.

- See code comments.  The extra hold in makeRoomForOutgoingCall was being
caused by the fact conferences do not have a target phoneaccount set.
This is causing makeRoomForOutgoingCall to hold the conference, when
the connection service is already doing that.

Bug: 22609387
Change-Id: Ieb4817bd35204c48cc5b7bf24384062afd5b7075