res_pjsip_sdp_rtp: Use negotiated DTMF Payload types on bitrate mismatch

res_pjsip_sdp_rtp: Use negotiated DTMF Payload types on bitrate mismatch

When Asterisk sends an offer to Bob that includes 48K and 8K codecs with
matching 4733 offers, Bob may want to use the 48K audio codec but can not
accept 48K digits and so negotiates for a mixed set.

Asterisk will now check Bob's offer to make sure Bob has indicated this is
acceptible and if not, will use Bob's preference.

Fixes: #847
(cherry picked from commit cf5a6435c2b135dd38413ded04cbd9ed9ff6fb53)