fix: set audio status on loaders when setting up media groups

[gesinger]

Although the audio status is set in onTrackChanged for media groups, and
the function is called when the media groups are first set up, the track
is not always considered changed. This means that for demuxed audio, the
main loader may still think it should be using its own audio itself,
leading to issues when crossing discontinuities (i.e., the main loader
will cross the discontinuity before waiting for the audio loader to be
ready, leading to audio timestamps that aren't correct).

This change ensures that the audio status is set on setup, regardless of
whether the track is considered changed. Subsequent changes are handled
in onTrackChanged.

Requirements Checklist

• Feature implemented / Bug fixed
• If necessary, more likely in a feature request than a bug fix
  • Unit Tests updated or fixed
  • Docs/guides updated
  • Example created (starter template on JSBin)
• Reviewed by Two Core Contributors

[codecov]

Codecov Report

Merging #1126 (073deaf) into main (a4af004) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1126      +/-   ##
==========================================
+ Coverage   86.20%   86.22%   +0.01%     
==========================================
  Files          39       39              
  Lines        9284     9289       +5     
  Branches     2126     2127       +1     
==========================================
+ Hits         8003     8009       +6     
+ Misses       1281     1280       -1     

Impacted Files	Coverage Δ
src/media-groups.js	98.97% <100.00%> (+0.01%)	⬆️
src/source-updater.js	94.51% <0.00%> (+0.32%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a4af004...073deaf. Read the comment docs.