Fix Low-Latency HLS VTT subtitle part loading

[robwalch]

This PR will...

• Fix LL-HLS VTT subtitle part loading
• Fix fragment selection when live subtitle playlist does not overlap with position
• Fix subtitle selection in asset players while primary is detached
• Remove dead code in VTT parser, and remove redundant code in subtitle-stream-controller

Why is this Pull Request needed?

These changes unify subtitle segment finding with audio and video, fixing issues with subtitle part and segment loading.

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

Low-latency live with VTT parts https://llhls-demo.ovenmediaengine.com/app/stream/llhls.m3u8

Resolves issues:

• #7460
• #7522
• #7557

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