SPARK-3744 [STREAMING] FlumeStreamSuite will fail during port contention

Since it looked quite easy, I took the liberty of making a quick PR that just uses `Utils.startServiceOnPort` to fix this. It works locally for me.

Author: Sean Owen <sowen@cloudera.com>

Closes #2601 from srowen/SPARK-3744 and squashes the following commits:

ddc9319 [Sean Owen] Avoid port contention in tests by retrying several ports for Flume stream
Remove compiler warning from TaskContext change.

Author: Reynold Xin <rxin@apache.org>

Closes #2602 from rxin/warning and squashes the following commits:

130186b [Reynold Xin] Remove compiler warning from TaskContext change.