Fix live frag finding after detach and re-attach past live window

[robwalch]

This PR will...

When looking for the next fragment to load, if the target position is less than the start of the playlist, reset startPosition and nextLoadPosition to liveSyncPosition. Setting nextLoadPosition ensures that the audio-stream-controller can properly evaluate the main buffer before SourceBuffers have been added to the MediaSource.

Why is this Pull Request needed?

audio-stream-controller exits before loading anything when main's nextLoadPosition is incorrect in some cases.

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

Resolves issues:

Fixes #5741

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