[FR] Open up FirebaseMessagingException

**Is your feature request related to a problem? Please describe.**
FirebaseMessagingException is final and has package private/private constructors. This prevents using the exception in tests. The firebase recommendation to deal with invalid tokens is to react on error code UNREGISTERED and INVALID_ARGUMENT. In order to test my code that deals with this scenario I would need to create a FirebaseMessagingException with the specified error codes which I can't.

**Describe the solution you'd like**
Open up FirebaseMessagingException to allow instantiation in tests.

Start by locating FirebaseMessagingException and reviewing its constructors and existing tests. Confirm that tests can instantiate exceptions for the UNREGISTERED and INVALID_ARGUMENT error codes while preserving the SDK's intended exception behavior; done when those scenarios are testable.

Scritto dal modello di indicizzazione a partire dal testo della issue.