RFR: 8359119: There are several opportunities to use Stable Values in j.n.c.Charset

[Per Minborg]

Fields and methods can better leverage the Stable Value API compared to using DCL and holder classes. There are also some fields that can be marked `@Stable`.

This PR passes tier1, tier2, and tier3 tests on multiple platforms.

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Commit messages:
 - Add stable value functionality

Changes: https://git.openjdk.org/jdk/pull/25727/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25727&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8359119
  Stats: 70 lines in 1 file changed: 12 ins; 29 del; 29 mod
  Patch: https://git.openjdk.org/jdk/pull/25727.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25727/head:pull/25727

PR: https://git.openjdk.org/jdk/pull/25727