Fix system video view leak

[defagos]

As a user I don't want playback to unnecessarily continue in the background after I have closed a player.

Acceptance criteria

• SystemVideoView does not leak resources anymore (FB11934227).
• The following works:
  • Open the Video URL example of theSystem player (using AVPlayer) section located under the Showcase tab.
  • Pause, then resume playback.
  • Close the player. Sound must correctly stop.

Tasks

• [x] Wait for Apple feedback and check whether the behavior has been fixed.
• [x] Remove warning mentioning FB11934227 in SystemVideoView documentation as well as in the getting started guide.
• [x] ~Mark SystemVideoView as being available starting from the iOS version it was fixed in.~