Fix loading of audio media options with the same URI as the current level

[robwalch]

This PR will...

Only load audio via the stream-controller (main playlist / active level) when the alternate audio rendition has the same URL.

Why is this Pull Request needed?

The stream-controller was not loading audio for audio-only streams when there were alternate audio options. It assumed that the main variant was inactive or represented by another alternate. However, if the current alternate has a (redundant) URL matching that of the main variant, instead of opting to omit the URL attribute, that alternate should not handled by the the audio-track and audio-stream-controllers.

Are there any points in the code the reviewer needs to double check?

Resolves issues:

Resolves #6859
Resolves #6783 (Fixed with #6785 but regressed with #6845)

Checklist

• changes have been done against master branch, and PR does not conflict
• new unit / functional tests have been added (whenever applicable)
• API or design changes are documented in API.md