RFR: 8261880: Change nested classes in java.base to static nested classes where possible

[Сергей Цыпанов]

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

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Commit messages:
 - 8261880: Change nested classes in java.base to static nested classes where possible

Changes: https://git.openjdk.java.net/jdk/pull/2589/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2589&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261880
  Stats: 20 lines in 16 files changed: 0 ins; 0 del; 20 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2589.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2589/head:pull/2589

PR: https://git.openjdk.java.net/jdk/pull/2589