RFR: 8320400: Make fields final in 'jdk.internal.util.xml.impl' package

[Andrey Turbanov]

A few classes in `jdk.internal.util.xml.impl` package have non-final fields which could easily be marked `final`.

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Commit messages:
 - [PATCH] Make fields final in jdk.internal.util.xml.impl package

Changes: https://git.openjdk.org/jdk/pull/15691/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15691&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320400
  Stats: 38 lines in 7 files changed: 0 ins; 0 del; 38 mod
  Patch: https://git.openjdk.org/jdk/pull/15691.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15691/head:pull/15691

PR: https://git.openjdk.org/jdk/pull/15691