Pull request

Description

This PR introduces a state object to handle the Picture-in-Picture state and interactions, following the approach used in Media3's ui-compose module.

Changes made

• Introduce a PictureInPictureButtonState.
• Update demo players that support Picture-in-Picture to use PictureInPictureButtonState.
• Update DemoPlayer to always display the Picture-in-Picture button. But it can be disabled if Picture-in-Picture is not supported. This follows what Media3 is doing in its Compose demo.
• Update the Readme to reflect the recent min SDK change.

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).