RFR: 8275240: Change nested classes in jdk.attach to static nested classes

[Andrey Turbanov]

Non-static classes hold a link to their parent classes, which in many cases can be avoided.
Similar cleanup in java.base - [JDK-8261880](https://bugs.openjdk.java.net/browse/JDK-8261880)

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Commit messages:
 - [PATCH] Change nested classes in jdk.attach to static nested classes where possible

Changes: https://git.openjdk.java.net/jdk/pull/5904/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5904&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275240
  Stats: 4 lines in 4 files changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5904.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5904/head:pull/5904

PR: https://git.openjdk.java.net/jdk/pull/5904