RFR: 8318569: Add getter methods for Locale and Patterns in ListFormat

[Naoto Sato]

Proposing adding getter methods for `locale` and `patterns` fields in the `ListFormat` instances. Those fields are used for `equals()`, but without the public getter methods, users cannot tell the reasoning of the equality of two instances. A corresponding CSR has also been drafted.

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Commit messages:
 - Simplified the method spec
 - initial commit

Changes: https://git.openjdk.org/jdk/pull/16293/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16293&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318569
  Stats: 60 lines in 2 files changed: 59 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/16293.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16293/head:pull/16293

PR: https://git.openjdk.org/jdk/pull/16293