res_pjsip_session: Rewrite o= with external_media_address.

PJSIP allows a domain name as external_media_address. This allows chan_pjsip to
be used behind a NAT with changing IP addresses. The IP address of that domain
is resolved to the c= line already. This change sets also the o= line to that
domain.

ASTERISK-27341 #close

Change-Id: I690163b6e762042ec38b3995aa5c9bea909d8ec4