Add carrier configuration options to allow enforcing max conference size.

Adding carrier config options to to allow enforcing a maxmimum size for
IMS conferences.
Also adding small performance enhancement to setConferenceables so that
if the list of conferenceable connections does not change it does not
send an update to listeners.

Test: Manual
Bug: 29548224
Change-Id: I109c91c4147814547bb75003d312d81d41a52c13