RFR: 8302858: Polish FlightRecorderMXBeanImpl

[Per Minborg]

This PR proposes some polishing of the `FlightRecorderMXBeanImpl` class.

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Commit messages:
 - Cleanup FlightRecorderMXBeanImpl

Changes: https://git.openjdk.org/jdk/pull/12656/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12656&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302858
  Stats: 11 lines in 1 file changed: 0 ins; 0 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/12656.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12656/head:pull/12656

PR: https://git.openjdk.org/jdk/pull/12656