RFR: 8302858: Polish FlightRecorderMXBeanImpl [v3]

[Per Minborg]

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

Per Minborg has updated the pull request incrementally with one additional commit since the last revision:

  Revoke key check

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12656/files
  - new: https://git.openjdk.org/jdk/pull/12656/files/c0949810..0d5ecaa1

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12656&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12656&range=01-02

  Stats: 3 lines in 1 file changed: 1 ins; 0 del; 2 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