[jdk18] RFR: 8278744: KeyStore:getAttributes() not returning unmodifiable Set

[Weijun Wang]

Make the return value of `PKCS12KeyStore::engineGetAttributes` immutable. Gather the `getAttributes()` value into a new `HashSet` and then make it immutable. This ensures the final result itself is not mutable and it also will not change even if the internal `entry.attributes` is modified.

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Commit messages:
 - 8278744: KeyStore:getAttributes() not returning unmodifiable Set

Changes: https://git.openjdk.java.net/jdk18/pull/23/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk18&pr=23&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278744
  Stats: 65 lines in 2 files changed: 64 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk18/pull/23.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk18 pull/23/head:pull/23

PR: https://git.openjdk.java.net/jdk18/pull/23