Keep alive improvements:

- Automatically send keep alive to camera once per minute; don't
  wait for it to send a ping to us first because that seems sporadic.
- Use audio/video protocol identifier when sending keep alive on
  that socket.  That it what we receive from the camera on that
  socket when it sends keep alive, e.g. when connected for Talk
  data but not sending anything.  Even though sending with the
  operation protocol identifier didn't seem to cause a problem.