[FR] Relax encapsulation for Firebase Messaging exception and Messages payload to enable testability

Currently, unit testing for integration with Firebase Messaging is very challenging. The encapsulation imposed by the final and visibility of the method makes it untestable. On top of this, there is no emulator for Firebase messaging which could help with testing integration with a more production-like environment.

1) I want to emulate exceptions on FCM Messaging like UNREGISTERED, INVALID_ARGUMENT when calling `com.google.firebase.messaging.FirebaseMessaging#send(com.google.firebase.messaging.Message)` to test my error handling implementation e.g deleting invalid token

- com.google.firebase.messaging.FirebaseMessagingException is final thus mocking with popular libraries like Mockito is more difficult. We need to enable extra feature flags in Mockito to allow for mocking the class
- It's impossible to create an instance of an exception with FCM Messaging exception as factory methods and constructors are package protected `com.google.firebase.messaging.FirebaseMessagingException#withMessagingErrorCode`

2) I want to unit test if a message with a given payalod was sent to FCM
Currently FCM payload represented with `com.google.firebase.messaging.Message` is not accessible to third parties via getters.

- Assuming I want to test if a proper payload was sent via `com.google.firebase.messaging.FirebaseMessaging#send(com.google.firebase.messaging.Message)` to FCM I would like to mock com.google.firebase.messaging.FirebaseMessaging and validate that `com.google.firebase.messaging.FirebaseMessaging#send(com.google.firebase.messaging.Message)` method was invoked with given `data` and `token`

In this case, developers used `@VisibleForTesting` for their purposes for testing while they totally forgot about developers who use their library and can not access the payload of the message. On top of this, there is no provided reasonable toString to make any assertions based on this.

**Describe the solution you'd like**
**Exceptions testing**
- Option 1
Relax encapsulation for com.google.firebase.messaging.FirebaseMessagingException by deleting "final" on class and make `errorCode` protected
- Option 2
Make factory method public `com.google.firebase.messaging.FirebaseMessagingException#withMessagingErrorCode`

**Messages testing**
- Option 1
Make getter public for `com.google.firebase.messaging.Message#Message` to allow accessing payload of FCM message
- Option 2
Provide reasonable toString to allow asserting on the toString method

**Describe alternatives you've considered**

- Provide emulator for FCM which allows emulating different scenarios including errors
- Provide dedicated implementations for testing purposes, open for all users

**Additional context**
Too aggressive encapsulation in my opinion is harmful. Developers should balance encapsulation and testability, taking into account the users of their libraries not only their own goals to protect their code with all means available in Java which are not perfect