## What changes were proposed in this pull request?

`Signaling.cancelOnInterrupt` leaks a SparkContext per call and it makes ReplSuite unstable.

This PR adds `SparkContext.getActive` to allow `Signaling.cancelOnInterrupt` to get the active `SparkContext` to avoid the leak.

## How was this patch tested?

Jenkins

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #16825 from zsxwing/SPARK-19481.