siptransport: fix tlsInfos_

sipTransport was only true on destroy, causing tlsInfos_ to be
always incorrect.
Moreover, if the SIPTransport is generated from a channeled socket,
we do not need to copy the peerCertificate.
Finally, update conference to use the tlsInfos_ to get the URI
instead of playing with peerNumber's

Change-Id: If8b93a4614e790e6ffbfd1bcc0aa68f6407344ce