Fix settings.advisory_script_uri in iOS createShell

[lext-7]

Fix settings.advisory_script_uri assignment in https://github.com/flutter/engine/blob/master/shell/platform/darwin/ios/framework/Source/FlutterEngine.mm#L283 and https://github.com/flutter/engine/blob/master/shell/platform/darwin/ios/framework/Source/FlutterEngine.mm#L286.

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[lext-7]

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