[jdk16u] RFR: 8260236: better init AnnotationCollector _contended_group

[Aleksey Shipilev]

This eliminates a potential source of bugs, and keeps codebases in sync (I see 11.0.12-oracle). Patch apples to 11u cleanly, passes tier{1,2}.

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Commit messages:
 - Backport fd2641ed36c8cdb59c33675bd9ed909b47cd002e

Changes: https://git.openjdk.java.net/jdk16u/pull/83/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk16u&pr=83&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260236
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk16u/pull/83.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk16u pull/83/head:pull/83

PR: https://git.openjdk.java.net/jdk16u/pull/83