[SPARK-16966][SQL][CORE] App Name is a randomUUID even when "spark.app.name" exists

## What changes were proposed in this pull request?

Don't override app name specified in `SparkConf` with a random app name. Only set it if the conf has no app name even after options have been applied.

See also https://github.com/apache/spark/pull/14602
This is similar to Sherry302 's original proposal in https://github.com/apache/spark/pull/14556

## How was this patch tested?

Jenkins test, with new case reproducing the bug

Author: Sean Owen <sowen@cloudera.com>

Closes #14630 from srowen/SPARK-16966.2.