Add simpler delegate method for FCM tokens (#375)

This new delegate method will be called generally once per app start, to always provide a current token. This token may change over time. This simpler method makes integration much simpler, as:

* Developers no longer have to check for a current token using the `.fcmToken` property, and also check for token changes using the `-messaging:didRefreshRegistrationToken:` delegate method.
* There is a single code path for when a token is available, making operations that depend on a token being available easier to implement. For example, this is the right method to always upload your FCM token to your application server, or to subscribe to topics, etc.

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