video_rtp_session: allow socket transmissions for response packets

Bitrate adjustment (REMB) packets in video calls are sent back on the
same socket pair as the video itself.
Video being enabled or disabled should thusly not stop communication
on these sockets fully as they could still be needed for these packets.

Change-Id: I3789aae44884ad5acfe5e1b014767ff2bf84ba33