[SPARK-6133] Make sc.stop() idempotent

Before we would get the following (benign) error if we called `sc.stop()` twice. This is because the listener bus would try to post the end event again even after it has already stopped. This happens occasionally when flaky tests fail, usually as a result of other sources of error. Either way we shouldn't be logging this error when it is not the cause of the failure.
```
ERROR LiveListenerBus: SparkListenerBus has already stopped! Dropping event SparkListenerApplicationEnd(1425348445682)
```

Author: Andrew Or <andrew@databricks.com>

Closes #4871 from andrewor14/sc-stop and squashes the following commits:

a14afc5 [Andrew Or] Move code after code
915db16 [Andrew Or] Move code into code