Add API for checking subscription status of a given topic

It is currently not possible to check whether a user is subscribed to a topic. This would be really nice to have in order to be able to show current state of subscription, for instance with a switch button.

The only current solutions seems to be keeping a state localy (with the risk of it being out of sync) or checking via our own server (which seems counter-intuitive since that's the only Firebase-related thing that would be in the server).

The implementation could just be `void isSubscribed(String topic)` or `List subscribedTopics()` depending on what i best for the SDK - both variants can solve the specific issue.

There is a similar feature request in the iOS SDK: firebase/firebase-ios-sdk#225

Start by locating the Android SDK module and existing topic-subscription entry points, then review the related iOS feature request linked in the issue. Decide which proposed public API fits the SDK and define done as allowing callers to determine a user's subscription status for a topic.

Written by the indexing model from the issue text.