FR: Add support for fetching topic subscriptions

Currently, `FirebaseMessaging` (and `InstanceIdClient` that it delegates to) provides support for subscribing to the topic and unsubscribing. However, to be able to fully (and reliably) manage topics from the server, these should be complemented with the ability to fetch the list of current topic subscriptions from the Firebase.

Apparently, the Instance ID API (i.e. `iid.googleapis.com/iid`) provides this capability - see https://developers.google.com/instance-id/reference/server#get_information_about_app_instances.

It would be nice if the SDK would expose this operation on the `InstanceIdClient`, and subsequently, `FirebaseMessaging`.