RFR: 8300177 : URISyntaxException fields can be final

[Darragh Clarke]

Made the two fields `input` and `index` final, this didn't need any test changes as far as I could tell so this PR is longer than the diff

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Commit messages:
 - made input and index fields final

Changes: https://git.openjdk.org/jdk/pull/12415/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12415&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300177
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/12415.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12415/head:pull/12415

PR: https://git.openjdk.org/jdk/pull/12415