Improve setting support

[defagos]

Description

Following a proposal made in #636 here is a variant which does not make the configuration mutable while still providing mutability for the audiovisual background policy.

Changes made

• Make audiovisualBackgroundPlaybackPolicy a computed property instead of an immutable player configuration setting.
• Remove external playback and audiovisual background policy from global app settings. External playback is now always available with our custom player and unavailable in some limited demos, which means we still can observe all behaviors in the demo app.
• Add audiovisual policy setting to the opt-in demo. Its layout has slightly been improved in the process (use of a List).

Checklist

• APIs have been properly documented (if relevant).
• The documentation has been updated (if relevant).
• New unit tests have been written (if relevant).
• The demo has been updated (if relevant).
• The playground has been updated (if relevant).