Fix system video view overlay issues

[defagos]

Description

This PR fixes two issues introduced in #430:

• The overlay was opaque, always hiding the content behind it.
• The app was crashing when transitioning from an inline system video view to its full screen counterpart (due to parent / child relationship inconsistency).

Changes made

• Remove parent / child relationship to avoid the crash.
• Make the overlay hosting controller view background trasnparent.
• Improve property naming.
• Make system video view coordinator available on all platforms (even if only doing meaningful work for tvOS at the moment).

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).