RFR: 8315850: Improve AbstractMap anonymous Iterator classes

[Per Minborg]

This PR proposes to slightly improve some iterators of `AbstractMap`:

* Code reuse
* A field declared `final`
* Add missing `@Override` annotations

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Commit messages:
 - Improve AbstractMap anonymous Iterator classes

Changes: https://git.openjdk.org/jdk/pull/15615/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15615&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315850
  Stats: 76 lines in 1 file changed: 44 ins; 28 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/15615.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15615/head:pull/15615

PR: https://git.openjdk.org/jdk/pull/15615