RFR: 8323624: ProviderList.ServiceList does not need to be a list

[Weijun Wang]

Re-implement it as an `Iterable` to be safe and make debugger happy.

No regression, just a refactoring.

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Commit messages:
 - the fix

Changes: https://git.openjdk.org/jdk/pull/17381/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17381&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8323624
  Stats: 56 lines in 12 files changed: 0 ins; 28 del; 28 mod
  Patch: https://git.openjdk.org/jdk/pull/17381.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17381/head:pull/17381

PR: https://git.openjdk.org/jdk/pull/17381