NM returns new SCHEDULED container status to older clients

YARN-4597 added a new SCHEDULED container state and that state is returned to clients when the container is localizing, etc. However the client may be running on an older software version that does not have the new SCHEDULED state which could lead the client to crash on the unexpected container state value or make incorrect assumptions like any state != NEW and != RUNNING must be COMPLETED which was true in the older version.