[SPARK-19793] Use clock.getTimeMillis when mark task as finished in TaskSetManager.

## What changes were proposed in this pull request?

TaskSetManager is now using `System.getCurrentTimeMillis` when mark task as finished in `handleSuccessfulTask` and `handleFailedTask`. Thus developer cannot set the tasks finishing time in unit test. When `handleSuccessfulTask`, task's duration = `System.getCurrentTimeMillis` - launchTime(which can be set by `clock`), the result is not correct.

## How was this patch tested?
Existing tests.

Author: jinxing <jinxing6042@126.com>

Closes #17133 from jinxing64/SPARK-19793.