[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

core/src/main/scala/org/apache/spark/SparkContext.scala

@@ -1392,10 +1392,10 @@ class SparkContext(config: SparkConf) extends Logging with ExecutorAllocationCli
   /** Shut down the SparkContext. */
   def stop() {
     SparkContext.SPARK_CONTEXT_CONSTRUCTOR_LOCK.synchronized {
-      postApplicationEnd()
-      ui.foreach(_.stop())
       if (!stopped) {
         stopped = true
+        postApplicationEnd()
+        ui.foreach(_.stop())
         env.metricsSystem.report()
         metadataCleaner.cancel()
         cleaner.foreach(_.stop())