CDIViewProvider.getViewBean should be protected rather than private

[vaadin-bot]

Originally by nahkh

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When implementing a more complicated view management system (in our case based on an xml file describing the view structure), it would be beneficial if we could override getViewBean with our own implementation. As it stands the we have essentially copy the entire class to extend it.

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Imported from https://dev.vaadin.com/ issue #18627