[SPARK-13992][CORE][PYSPARK][FOLLOWUP] Update OFF_HEAP semantics for Java api and Python api

## What changes were proposed in this pull request?

- updated `OFF_HEAP` semantics for `StorageLevels.java`
- updated `OFF_HEAP` semantics for `storagelevel.py`

## How was this patch tested?

no need to test

Author: Liwei Lin <lwlin7@gmail.com>

Closes #12126 from lw-lin/storagelevel.py.

core/src/main/java/org/apache/spark/api/java/StorageLevels.java

@@ -34,7 +34,7 @@ public class StorageLevels {
   public static final StorageLevel MEMORY_AND_DISK_2 = create(true, true, false, true, 2);
   public static final StorageLevel MEMORY_AND_DISK_SER = create(true, true, false, false, 1);
   public static final StorageLevel MEMORY_AND_DISK_SER_2 = create(true, true, false, false, 2);
-  public static final StorageLevel OFF_HEAP = create(false, false, true, false, 1);
+  public static final StorageLevel OFF_HEAP = create(true, true, true, false, 1);
 
   /**
    * Create a new StorageLevel object.

python/pyspark/storagelevel.py

@@ -55,7 +55,7 @@ StorageLevel.MEMORY_ONLY = StorageLevel(False, True, False, False)
 StorageLevel.MEMORY_ONLY_2 = StorageLevel(False, True, False, False, 2)
 StorageLevel.MEMORY_AND_DISK = StorageLevel(True, True, False, False)
 StorageLevel.MEMORY_AND_DISK_2 = StorageLevel(True, True, False, False, 2)
-StorageLevel.OFF_HEAP = StorageLevel(False, False, True, False, 1)
+StorageLevel.OFF_HEAP = StorageLevel(True, True, True, False, 1)
 
 """
 .. note:: The following four storage level constants are deprecated in 2.0, since the records \