[SPARK-13937][PYSPARK][ML] Change JavaWrapper _java_obj from static to member variable

## What changes were proposed in this pull request?
In PySpark wrapper.py JavaWrapper change _java_obj from an unused static variable to a member variable that is consistent with usage in derived classes.

## How was this patch tested?
Ran python tests for ML and MLlib.

Author: Bryan Cutler <cutlerb@gmail.com>

Closes #11767 from BryanCutler/JavaWrapper-static-_java_obj-SPARK-13937.

python/pyspark/ml/wrapper.py

@@ -34,10 +34,15 @@ class JavaWrapper(Params):
 
     __metaclass__ = ABCMeta
 
-    #: The wrapped Java companion object. Subclasses should initialize
-    #: it properly. The param values in the Java object should be
-    #: synced with the Python wrapper in fit/transform/evaluate/copy.
-    _java_obj = None
+    def __init__(self):
+        """
+        Initialize the wrapped java object to None
+        """
+        super(JavaWrapper, self).__init__()
+        #: The wrapped Java companion object. Subclasses should initialize
+        #: it properly. The param values in the Java object should be
+        #: synced with the Python wrapper in fit/transform/evaluate/copy.
+        self._java_obj = None
 
     @staticmethod
     def _new_java_obj(java_class, *args):