res_pjsip_refer/chan_sip: Fix INVITE with replaces transfer to ConfBridge

There is a problem when an INVITE-with-Replaces transfer targets a channel
in a ConfBridge.  The transfer will unconditionally swap out the
ConfBridge channel.  Unfortunately, the ConfBridge state will not be aware
of this change.  Unexpected behavior will happen as a result since
ConfBridge channels currently can only be replaced by a masquerade and not
normal bridge channel moves.

* We just need to pretend that the channel isn't in a bridge (like other
transfer methods already do) so the transfer channel will masquerade into
the ConfBridge channel.

Change-Id: I209beb0e748fa4f4b92a576f36afa8f495ba4c82