chan_sip: SDP: Sidestep stream parsing when its media is disabled.

Previously, chan_sip parsed all known media streams in an SDP offer
like video (and text) even when videosupport=no (and textsupport=no).
This wasted processor power. Furthermore, chan_sip accepted SDP offers,
including no audio but just video (or text) streams although
videosupport=no (or textsupport=no). Finally, chan_sip denied the whole
offer instead of individual streams when they had encryption (SDES-sRTP)
unexpectedly enabled.

ASTERISK-29238
ASTERISK-29237
ASTERISK-29222

Change-Id: Ie49e4e2a11f0265f914b684738348ba8c0f89755