[SPARK-13823][SPARK-13397][SPARK-13395][CORE] More warnings, StandardCharset follow up

## What changes were proposed in this pull request?

Follow up to https://github.com/apache/spark/pull/11657

- Also update `String.getBytes("UTF-8")` to use `StandardCharsets.UTF_8`
- And fix one last new Coverity warning that turned up (use of unguarded `wait()` replaced by simpler/more robust `java.util.concurrent` classes in tests)
- And while we're here cleaning up Coverity warnings, just fix about 15 more build warnings

## How was this patch tested?

Jenkins tests

Author: Sean Owen <sowen@cloudera.com>

Closes #11725 from srowen/SPARK-13823.2.