RFR: 8302858: Polish FlightRecorderMXBeanImpl [v2]

[Per Minborg]

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

Per Minborg has updated the pull request incrementally with two additional commits since the last revision:

 - Remove typo
 - Rollback value check

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/12656/files
  - new: https://git.openjdk.org/jdk/pull/12656/files/4ccfcb1a..c0949810

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12656&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12656&range=00-01

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