Race between member variable being accessed due to leaking uninitialized state via OutboundObserverFactory

The OutboundObserverFactory leaks the InboundObserver and makes it ready to start getting requests before the method returns leading to a dangerous state where the passed in InboundObserver could be used before the constructor of the enclosing class is finished.

This was found when running the Spark PVR PostCommit:

Looks like this went from being a flake to a hard failure:
https://ci-beam.apache.org/job/beam_PostCommit_Java_PVR_Spark_Batch/
https://ci-beam.apache.org/job/beam_PostCommit_Java_PVR_Spark_Batch/5009/

18:41:18 Build timed out (after 100 minutes). Marking the build as aborted.