NullPointerException when accessing TorControlConnection too early

When a user of `TorService` starts and binds to the service, then ([as documented](https://github.com/guardianproject/tor-android/blob/c5d5b2c00876cbf2e697668dd14ff6021ea18c7b/tor-android-binary/src/main/java/org/torproject/jni/TorService.java#L34)) tries to use the `TorControlConnection`, it can happen that it hasn't been assigned and a `NullPointerException` is thrown. This is because the `TorControlConnection` gets only assigned [in another thread after doing some extra work](https://github.com/guardianproject/tor-android/blob/c5d5b2c00876cbf2e697668dd14ff6021ea18c7b/tor-android-binary/src/main/java/org/torproject/jni/TorService.java#L262).

How is a user of the library supposed to know when the control connection becomes available? Are they expected to register a broadcast receiver and wait for the `TorService.STATUS_ON` event? That would be quite a cumbersome, but possible solution. If it is that, it should at least get documented.