RFR: 8300176: URLEncoder/URLDecoder static fields should be private static final

[Darragh Clarke]

Made the `static` fields `private static final`, updated the naming as well to reflect this.

This meant I updated `URLDecoder` to set the `DEFAULT_ENCODING_NAME` itself since it had been previously getting it from `URLEncoder`.

Since these fields are only referenced from inside the classes nothing else needed updated

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Commit messages:
 - updated fields to be private static final, updated naming to match

Changes: https://git.openjdk.org/jdk/pull/12122/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12122&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300176
  Stats: 28 lines in 2 files changed: 6 ins; 0 del; 22 mod
  Patch: https://git.openjdk.org/jdk/pull/12122.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12122/head:pull/12122

PR: https://git.openjdk.org/jdk/pull/12122