Replace RtpDemuxerCriteria member with discrete variables

Instead of keeping an RtpDemuxerCriteria object as a member, replace it
with distinct member variables (mid_, payload_types_, ssrcs_) inside
BaseChannel. The RtpDemuxerCriteria object is now constructed on the
stack, on the network thread, when needed (a step that clarifies the
intended threading slightly).

This disentangles the mid, which is const and must be readable from the
worker and network threads, from other demuxer properties. The channel
implementations also only use the ssrcs and payload_types properties of
the RtpDemuxerCriteria, don't need the full object and now there is not
a duplicate copy of payload_types_.

Those changes unlock the ability to further move parts of the channel
implementation over to the network thread and get rid of some of the
thread hopping.

Some more work needs to be done to fully move the demuxer state to the
network thread however.

Bug: webrtc:42222117
Change-Id: I42f8861c0ed04a8d19656edba7f0053142f07b06
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/456060
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47144}