[MSE,HLS] Add audio/video config map getters to MediaTrack

To assist upcoming HLS demuxer logic operation amongst potentially
multiple audio/video configs parsed from a bytestream initialization
segment, this change adds getters for the audio and video config maps
reported by StreamParsers via their new-config-callback when they finish
parsing an initialization segment.

This change is split off from the HLS Demuxer WIP CL begun by
cassew@chromium.org [1], and includes some additional minor test updates
that exercise the new getters.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/3425659

Bug: 1266991
Change-Id: Ic3ac98453f27685c148163dcb325cf1736a011ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4017965
Commit-Queue: Will Cassella <cassew@chromium.org>
Reviewed-by: Will Cassella <cassew@chromium.org>
Auto-Submit: Matthew Wolenetz <wolenetz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1069572}