RFR: 8315850: Improve AbstractMap anonymous Iterator classes [v3]

[Per Minborg]

> This PR proposes to slightly improve some iterators of `AbstractMap`:
> 
> * Code reuse
> * A field declared `final`
> * Add missing `@Override` annotations

Per Minborg has updated the pull request incrementally with two additional commits since the last revision:

 - Fix additional formating issue
 - Don't use polymorphism and reformat code

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/15615/files
  - new: https://git.openjdk.org/jdk/pull/15615/files/aab2472a..1057e166

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15615&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15615&range=01-02

  Stats: 30 lines in 1 file changed: 0 ins; 17 del; 13 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