[Unified Plan] Fix issues with recycling m= sections

Previously, the PeerConnection would look at the pending local
and remote descriptions also to determine if an m= section is
recycled. That is not quite spec compliant and breaks down under
some edge cases. This changes the PeerConnection to look only at
the *current* local or remote description (i.e., the descriptions
from the last time the PeerConnection was in a stable signaling
state) to determine if an m= section is recycled.

Additionally, the MediaSessionFactory only looked at the local
description to determine if an m= section is recycled. The full
criteria requires looking at the current local and current remote
m= sections. This change adds a state enum to the
MediaDescriptionOptions so that the MediaSessionFactory knows if
a media section is being recycled without duplicating the logic
in PeerConnection.

Tests are added to cover additional edge cases.

Bug: chromium:899680
Change-Id: I5bcf0f88957a61653269ed8bb50b2018500bc1d5
Reviewed-on: https://webrtc-review.googlesource.com/c/111293
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Seth Hampson <shampson@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25959}