Only turn on or off recording if explicitly specified
When we receive an offer or an answer, we will only turn recording on if
it contains the "record-call=yes" flag. Likewise, we only turn recording
off if it contains the "record-call=no" flag. If no flag is specified,
the call keeps its current recording status. It used to be the case that
not specifying "record-call=yes" would turn off call recording.
This fix is necessary to account for SDP renegotiation during
hold/unhold. If the call system sends over another offer during SDP
renegotiation to hold and then unhold, we don't want to change recording
behavior unless the call system specifically says so.