Improve Picture in Picture behavior when several video views are involved

[defagos]

Description

This PR improves Picture in Picture (PiP) behavior when several VideoViews are involved. This fixes issues such as:

• VideoViews being stolen when PiP is enabled for other views.
• PiP being unexpectedly terminated.

Implementation considerations

The work required to make PiP more reliable in the above scenarios was not trivial and involved several smaller steps:

1. Ensure that a VideoView supporting PiP is reassigned a video layer when stolen by another view enabled for PiP.
2. Consider player identity to find the best match when restoring a video view from a PiP-enabled video layer.
3. Store the list of video layers supporting PiP so that PiP can fallback onto a formerly enabled view.
4. Since the player identity is considered when finding a best match (previously the 1st video view was always winning), two additional use cases have to be considered when PiP ends:
   • No PiP-enabled view is visible, in which case PiP-resources must be cleaned up.
   • PiP-enabled views are visible, but for other players. In this case the most recently PiP-enabled player must be used as fallback.

Changes made

• Update PiP controller using content sources (works well and avoids recreating controllers and using KVO).
• Increase restoration animation delay to mitigate improve restoration animations in some scenarios.
• Add demos to help test PiP behavior in the following cases (including toggling on / off PiP for a view at any time):
  • Twins view (same player whose content is displayed in two sibling views).
  • Multi view (two players with two associated views).
  • Transition view (one player whose content is displayed by a view, from which a modal displaying the same content can be opened).
• Update demo close button to make interaction with it more reliable.

Checklist

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