RFR: 8348351: Improve lazy initialization of the available currencies set [v2]

[Justin Lu]

> Please review this PR which stems from discussion in the PR of JDK-8347949.
> 
> The set of lazily loaded Currencies is a good fit for stable values. `available` can now be made final as it is a SV supplier.
> 
> (This change also includes an unrelated minor edit to comments in `Currency::getCurrencies` to make the distinction for simple vs special Currencies.)

Justin Lu has updated the pull request incrementally with one additional commit since the last revision:

  Chen's review - method name clarification

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Changes:
  - all: https://git.openjdk.org/jdk/pull/25134/files
  - new: https://git.openjdk.org/jdk/pull/25134/files/f9e9c6b2..e2672ba1

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=25134&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=25134&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/25134.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25134/head:pull/25134

PR: https://git.openjdk.org/jdk/pull/25134