## 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.