## What changes were proposed in this pull request?
Scala traits are difficult to maintain binary compatibility on, and as a result we had to introduce JavaSparkListener. In Spark 2.0 we can change SparkListener from a trait to an abstract class and then remove JavaSparkListener.

## How was this patch tested?
Updated related unit tests.

Author: Reynold Xin <rxin@databricks.com>

Closes #12142 from rxin/SPARK-14358.