Default the Fully Kiosk output codec to AAC (#6112)

# What does this implement/fix?

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Fully Kiosk aborts a long-lived MP3 stream after ~8m45s with "Failed
loading sound: Wrong URL or unsupported format?" and then re-requests
the stream url, so radio stations and long queues never play through.
The server logs no error at that point - the client simply gives up and
reconnects - and the reporter's A/B test on the same hardware showed AAC
playing for over two hours where MP3 reproducibly failed within 9
minutes.

Switch the provider default from MP3 to AAC. The entry stays visible and
overridable rather than hidden/enforced, since Fully Kiosk runs on a
wide range of Android versions. Config values equal to their default are
not persisted, so existing players that never explicitly picked a codec
pick this up too. As a bonus AAC should be same quality for less
bandwidth.

**Related issue (if applicable):**

- related issue https://github.com/music-assistant/support/issues/6248

## Types of changes

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- [X] Bugfix (non-breaking change which fixes an issue) — `bugfix`
- [ ] New feature (non-breaking change which adds functionality) —
`new-feature`
- [ ] Enhancement to an existing feature — `enhancement`
- [ ] New music/player/metadata/plugin provider — `new-provider`
- [ ] Breaking change (fix or feature that would cause existing
functionality to not work as expected) — `breaking-change`
- [ ] Refactor (no behaviour change) — `refactor`
- [ ] Documentation only — `documentation`
- [ ] Maintenance / chore — `maintenance`
- [ ] CI / workflow change — `ci`
- [ ] Dependencies bump — `dependencies`

## Checklist

- [X] The code change is tested and works locally.
- [X] `pre-commit run --all-files` passes.
- [X] `pytest` passes, and tests have been added/updated under `tests/`
where applicable.
- [ ] For changes to shared models, the companion PR in
`music-assistant/models` is linked.
- [ ] For changes affecting the UI, the companion PR in
`music-assistant/frontend` is linked.
- [X] I have read and complied with the project's [AI
Policy](https://github.com/music-assistant/.github/blob/main/AI_POLICY.md)
for any AI-assisted contributions.
- [ ] I have [raised a PR against the documentation
repository](https://github.com/music-assistant/music-assistant.io/blob/main/CONTRIBUTING.md)
targeting the main or beta branch as appropriate.

Co-authored-by: Claude <[email protected]>