[SPARK-4504][Examples] fix run-example failure if multiple example jars present

Pick example jar latest by timestamp

Author: Venkata Ramana G <ramana.gollamudi@huawei.com>
How do you know this will pick up the right jar?

More generally, I think the only situation someone will run into this is when switching between branches and rebuilding without cleaning. Not sure it's worth it to add workarounds in the code for that.

You'll get a similar error from `compute-classpath.sh` if multiple assembly jars exist. The same behavior should be just fine for the examples.