Pull request

Description

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

Changes made

• Introduce a FullscreenButtonState.
• Update the demo player to use FullscreenButtonState.
• Remove the ShowSystemUi Composable.

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).