[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.

首先定位 FirebaseMessagingException，并检查其构造函数和现有测试。确认测试可以为错误代码 UNREGISTERED 和 INVALID_ARGUMENT 实例化异常，同时保留 SDK 预期的异常行为；当这些场景可测试时即完成。

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