RFR: 8314877: Make fields final in 'java.net' package

[Andrey Turbanov]

A few classes in `java.net` package have non-final fields which could easily be marked `final`.

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Commit messages:
 - [PATCH]: Make fields final java.net

Changes: https://git.openjdk.org/jdk/pull/15358/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15358&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314877
  Stats: 31 lines in 10 files changed: 3 ins; 7 del; 21 mod
  Patch: https://git.openjdk.org/jdk/pull/15358.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15358/head:pull/15358

PR: https://git.openjdk.org/jdk/pull/15358