Option to keep display on when fullscreen

[Silic0nS0ldier]

This PR adds an argument keepDisplayOn to enterFullscreen.

• On Android, the fullscreen activity will set the FLAG_KEEP_SCREEN_ON flag.
• On iOS, the idle timer is disabled (UIApplication.shared.isIdleTimerDisabled = false)

The example app has been updated so that;

• The "keep display on" behaviour can be toggled.
• Individual videos can be more easily identified (previously it was a wall of black, save for the currently playing video).

Android has been tested (Pixel 8), iOS has not.

The intent behind this change is to keep the display on when viewing videos in fullscreen within the Bluesky mobile app.

[haileyok]

Thank you @Silic0nS0ldier! Going to get through this today, really appreciate you digging through here and getting this in 🙏

[haileyok]

This looks great! Thank you so much!