Fix #421 Don't expose css className for `u-react-spectrum-screenReaderOnly` as API (#423)
* Fix #421 Don't expose css className for `u-react-spectrum-screenReaderOnly` as API
We shouldn't expose our css classes as API. Instead we should provide components that apply them.
https://github.com/adobe/react-spectrum/pull/420/files#r298298301
* Fix #421 VisuallyHidden should be a public component rather than a utility
* Use dive() rather than shallow() in enzyme unit tests