[SPARK-12834][ML][PYTHON][BACKPORT] Change ser/de of JavaArray and JavaList

Backport of SPARK-12834 for branch-1.6

Original PR: #10772

Original commit message:
We use `SerDe.dumps()` to serialize `JavaArray` and `JavaList` in `PythonMLLibAPI`, then deserialize them with `PickleSerializer` in Python side. However, there is no need to transform them in such an inefficient way. Instead of it, we can use type conversion to convert them, e.g. `list(JavaArray)` or `list(JavaList)`. What's more, there is an issue to Ser/De Scala Array as I said in https://issues.apache.org/jira/browse/SPARK-12780

Author: Xusen Yin <[email protected]>

Closes #10941 from jkbradley/yinxusen-SPARK-12834-1.6.

Author: yinxusen

mllib/src/main/scala/org/apache/spark/mllib/api/python/PythonMLLibAPI.scala

@@ -1464,7 +1464,11 @@ private[spark] object SerDe extends Serializable {
   initialize()
 
   def dumps(obj: AnyRef): Array[Byte] = {
-    new Pickler().dumps(obj)
+    obj match {
+      // Pickler in Python side cannot deserialize Scala Array normally. See SPARK-12834.
+      case array: Array[_] => new Pickler().dumps(array.toSeq.asJava)
+      case _ => new Pickler().dumps(obj)
+    }
   }
 
   def loads(bytes: Array[Byte]): AnyRef = {