Add backward-compatible KafkaSystemProducer constructor for Java clients

SAMZA-911 made a change to the constructor of KafkaSystemProducer.
While that change is backward-compatible for Scala clients, it is not for Java clients.

Although KafkaSystemProducer is not part of our public API, we have a team at LI that uses it in Java and we want to avoid breaking their code on a minor version bump.