Bug 1828517 (MOZ) - Improve rtp packet delivery through MediaPipeline and conduits. r=webrtc-reviewers,dbaker

With earlier fixes to upstream's PacketReceiver::DeliverPacket, received rtp
packets were parsed twice:
- In MediaPipeline, with the proper extension map in order to extract the header
  for filtering.
- In conduits, with no extension map.

The conduits would therefore not parse any rtp header extensions, leading to
problems later on as data from the extensions would not get exposed to
libwebrtc or js.

This patch removes the rtp packet parsing in the conduits and moves any setting
of metadata over to MediaPipeline so there is a single point responsible for
parsing rtp packets. The packet is then transferred verbatim via the conduits to
libwebrtc.

Differential Revision: https://phabricator.services.mozilla.com/D177536