RFR: 8309667: TLS handshake fails because of ConcurrentModificationException in PKCS12KeyStore.engineGetEntry

[Weijun Wang]

A different fix after https://github.com/openjdk/jdk/pull/14506 was closed.

Still haven't made the attributes set immutable but at least it is populated before an entry is added to `entries` and it will never be modified later.

I tried the newly added `AttributesMultiThread.java` test hundreds of times and only observed failures before this fix (~%2 failure rate).

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Commit messages:
 - remove x bits
 - the fix

Changes: https://git.openjdk.org/jdk/pull/15909/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15909&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309667
  Stats: 258 lines in 3 files changed: 250 ins; 2 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/15909.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15909/head:pull/15909

PR: https://git.openjdk.org/jdk/pull/15909