Accessibility for subtitles and audio tracks

[defagos]

As a user who requires accessibility features I want the best possible playback experience tailored to my needs.

Acceptance criteria

• Audio description tracks are favored if the corresponding accessibility setting has been enabled.
• SDH subtitles are favored if the corresponding accessibility setting has been enabled. The user experience is identical to Apple system player UI, i.e. the list does not present SDH and non-SDH subtitles were redundant.

Documentation

• Team discussion.

Tasks

• Provide AD audio tracks first if enabled in accessibility settings.
• Provide SDH subtitles first if enabled in accessibility settings. Match the result obtained for the system player and our custom menu with Apple Dolby Atmos sample stream.
• Document stream packaging recommendations (characteristics and autoselect).

[defagos]

Likely already implemented in #502 since the system automatically takes into account these settings.

[defagos]

AD already works well but SDH still needs some work so that the result is identical to the system player UI. Likely something very similar as support for forced subtitles, relying on MACaptionAppearanceCopyPreferredCaptioningMediaCharacteristics.

[defagos]

I implemented the feature on the sdh-support branch but this branch relies on #533 being merged. Once this PR has been merged we can open the issue branch and cherry-pick the commits.

The idea is to rely on Media Accessibility preferences to prefer CC / SDH when enabled. In this case the normal subtitles are replaced but, if only normal subtitles are available for some language, those will be returned instead. This is what the Apple official player UI does.

We have a slight difference with Apple implementation, which shows CCs when the Accesssibility CC+ SDH flag has not been enabled. This can be observed using the Apple Dolby Atmos sample stream. IMHO our implementation is better since it matches the setting description so I recommend we keep this difference.

Writing tests is a nightmare, though. I don't see how we can achieve something meaningful but this is something we can discuss next week with @waliid.

[defagos]

As for forced subtitles changes made on the Apple TV are partially reflected to the device but there is nothing we can do to fix this issue. It is still a minor issue IMHO.

[defagos]

Play CH test streams have only public.accessibility.describes-music-and-sound and won't therefore be correctly filtered based on the SDH / CC accessibility setting. Packaging should be updated to add the public.accessibility.transcribes-spoken-dialog as well.