RFR: 8348308: Make fields of ListSelectionEvent final

[Alexey Ivanov]

The fields of `ListSelectionEvent` are set in the constructor and are never modified, mark the fields — `firstIndex`, `lastIndex`, `isAdjusting` — with the `final` modifier.

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Commit messages:
 - 8348308: Make fields of ListSelectionEvent final

Changes: https://git.openjdk.org/jdk/pull/23243/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23243&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8348308
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/23243.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23243/head:pull/23243

PR: https://git.openjdk.org/jdk/pull/23243