[CORE]codeStyle: uniform ConcurrentHashMap define in StorageLevel.scala with other places

Author: Zhang, Liye <liye.zhang@intel.com>

Closes #2793 from liyezhang556520/uniformHashMap and squashes the following commits:

5884735 [Zhang, Liye] [CORE]codeStyle: uniform ConcurrentHashMap define in StorageLevel.scala

core/src/main/scala/org/apache/spark/storage/StorageLevel.scala

@@ -18,6 +18,7 @@
 package org.apache.spark.storage
 
 import java.io.{Externalizable, IOException, ObjectInput, ObjectOutput}
+import java.util.concurrent.ConcurrentHashMap
 
 import org.apache.spark.annotation.DeveloperApi
 import org.apache.spark.util.Utils
@@ -220,8 +221,7 @@ object StorageLevel {
     getCachedStorageLevel(obj)
   }
 
-  private[spark] val storageLevelCache =
-    new java.util.concurrent.ConcurrentHashMap[StorageLevel, StorageLevel]()
+  private[spark] val storageLevelCache = new ConcurrentHashMap[StorageLevel, StorageLevel]()
 
   private[spark] def getCachedStorageLevel(level: StorageLevel): StorageLevel = {
     storageLevelCache.putIfAbsent(level, level)