Bug 1041124 - [Messages][Refactoring] Make use of EventDispatcher in MessageManager. r=julien

Having said that, I'm in doubt whether "returns a rejected promise if there is no API" test case is real one. Can we have mozMobileMessage available, but one of its methods isn't visible? If yes, then I understand, otherwise I'd rather throw exception in MessageManager.init if mozMobileMessage isn't available, what do you think?