Fix up some of chan_sip's usage of the RTP engine API.

The get_local_address() function for an RTP instance was used when building an
SDP, but the results were not honored.  The RTP engine activate() function was
not being used once we have determined that media will now flow.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@250917 65c4cc65-6c06-0410-ace0-fbb531ad65f3