Support connections which do not allow conferencing.

Check carrier config in TelecomAccountRegistry when registering
PhoneAccounts.  When creating new TelephonyConnections, set a propery
on the connection to indicate whether it supports conferencing.  This is
then used in the conference controllers to determine whether a particular
connection can be in a conference with another connection or not.

Bug: 22864941
Change-Id: I199cbc1c9ad575732c5601a9bb587c8bca4815da
5 files changed