RFR: 8358318: JFR: Tighten up PlatformTracer initialization

[Aleksey Shipilev]

Started as SonarCloud complaint that MethodSetting.apply initializes a static field. But I think there is a larger problem here, which I completely missed during [JDK-8352738](https://bugs.openjdk.org/browse/JDK-8352738) review: the `PlatformTracer` initialization can technically run several times, AFAICS. This PR tightens up the initialization. It also allows us to call `ensureInitialized` from other places, if we ever find a need.

Additional testing:
 - [x] Linux x86_64 server fastdebug, `jdk_jfr`

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/25584/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25584&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8358318
  Stats: 21 lines in 2 files changed: 15 ins; 5 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/25584.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25584/head:pull/25584

PR: https://git.openjdk.org/jdk/pull/25584