Make FlutterException constructor public

[xegrox]

The default constructor access modifier in java is package-private, which restricts access outside of this package
Change the constructor to public so that others may construct their own FlutterException

[flutter-dashboard]

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[stuartmorgan-g]

I'll follow up in the Pigeon PR, but I don't think this is the right avenue here; the documentation for this class is "Thrown to indicate that a Flutter method invocation failed on the Flutter side", and you are trying to repurpose it for the opposite.