Add a way to determine dimensions of system UI

[krisgiesing]

This request came from Craig Hammel, asking for a way to determine the notification bar height on Android.

More generally, we want to provide a way to distinguish between the screen dimensions that can be drawn into, from the screen dimensions that are not obscured by other content. This includes system UI both on Android and on iOS, and potentially also overscan regions if we ever want to target TV. Note that the amount of overlap from system UI will depend on OS version and device type, so it's more complex than a hard coded constant.

[abarth]

This value is ui.window.padding.

[krisgiesing]

Ah, I didn't know about ui.window.padding. Thanks, closing.

[github-actions]

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