RFR: 8275342: Change nested classes in java.prefs to static nested classes

[Andrey Turbanov]

Non-static classes hold a link to their parent classes, which can be avoided.

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Commit messages:
 - [PATCH] Change nested classes in java.prefs to static nested classes

Changes: https://git.openjdk.java.net/jdk/pull/5971/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5971&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275342
  Stats: 20 lines in 3 files changed: 1 ins; 11 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5971.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5971/head:pull/5971

PR: https://git.openjdk.java.net/jdk/pull/5971