[SPARK-3797] Minor addendum to Yarn shuffle service

I did not realize there was a `network.util.JavaUtils` when I wrote this code. This PR moves the `ByteBuffer` string conversion to the appropriate place. I tested the changes on a stable yarn cluster.
Yeah maybe. It's just that it's not super intuitive why we need to use `JavaUtils` in scala code when we could have used something from Spark's `Utils`. I'd rather err on the safe side of being verbose. Maybe I'll reword the comment a little to make it sound less stupid.