[SPARK-3543] Clean up Java TaskContext implementation.

This addresses some minor issues in https://github.com/apache/spark/pull/2425

Author: Reynold Xin <rxin@apache.org>

Closes #2557 from rxin/TaskContext and squashes the following commits:

a51e5f6 [Reynold Xin] [SPARK-3543] Clean up Java TaskContext implementation.