This change eliminates two previous assumptions:
1. If a Flutter view is embedded in a parent view, that the parent view
controller would apply any status bar padding necessary.
2. That we should not apply padding unless the flutter view is
fullscreen.
A simple case where the first assumption fails to hold is a Flutter view
embedded in a UINavigationController with a hidden toolbar. A simple
case where the second assumption fails to hold is a view in a window
whose top overlaps the status bar but isn't fullscreen.
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