Update RenditionManager to use features of RenditionGroup

The RenditionGroup class has the ability to handle selecting similar
renditions, as well as map renditions by MediaTrack. We can use these
structures instead of the VariantID and RenditionID implementation,
which saves a lot of extra work.

As far as test changes go, there's some new ones that handle explicit
audio-only cases that were missing, and the only behavioral change is
that deselecting a preferred audio stream does not switch back to the
default one now. This isn't (yet) user facing, since not only is track
switching still behind --enable-experimental-web-platform-features, but
no additional audio tracks were being exposed anyway.

Bug: 361853710

Change-Id: I9fd5d860629c582e49f053e29f5f70877ac46dab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6463711
Reviewed-by: Frank Liberato <liberato@chromium.org>
Commit-Queue: Ted (Chromium) Meyer <tmathmeyer@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1450155}