Turn Priority into a non-extendable interface

[Jonathing]

This one is more of a nitpick/personal preference than anything else. The primary reason for this is to remove the boilerplate for the fields of the byte fields in the Priority class, but it also aligns with how the other interfaces in the EventBus API primarily convey information about the types the consumer can work with.

To preserve non-extensibility, I've created an empty internal interface simply named UtilityInterface, whose name entails that Priority is a utility interface and not an actual type that can be interacted with.

[LexManos]

Not really a fan of this. Ya some boiler plate is annoying but i think this is worse.

[PaintNinja]

Not a fan either. For newbie Java devs they may not yet know that those are static fields and try to find a way to get an instance, then go to the UtilityInterface and see it's public and think it needs to be implemented or something.