RFR: 8260448: Simplify ManagementFactory$PlatformMBeanFinder

[Claes Redestad]

Simplify and desugar the static initialization of PlatformMBeanFinder. 

While arguably making the code easier to comprehend, this saves 5-6ms on startup in various applications such as the spring-petclinic.

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Commit messages:
 - Simplify PlatformMBeanFinder

Changes: https://git.openjdk.java.net/jdk/pull/2244/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2244&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260448
  Stats: 64 lines in 1 file changed: 21 ins; 26 del; 17 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2244.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2244/head:pull/2244

PR: https://git.openjdk.java.net/jdk/pull/2244