SDP: Explicitly stop a RTP instance before destoying it.

* Made sdp_add_m_from_rtp_stream() and sdp_add_m_from_udptl_stream()
handle generating disabled/declined streams.

* Added /main/sdp/sdp_merge_asymmetric unit test.  It currently does not
check the offerer side negotiated SDP because that isn't the purpose of
this patch and there is much to be done to handle declined/dummy streams.

* Added T.38 image streams to the /main/sdp/sdp_merge_symmetric and
/main/sdp/sdp_merge_crisscross unit tests.

Change-Id: Ib4dcb3ca4f9a9133b376f4e3302f9a1f963f2b31