RFR: 8297275: WatchService delay with PollingWatchService causes test failures

[Tyler Steele]

This minor change fixes a compliance test failure which was partially addressed by https://github.com/openjdk/jdk/commit/1bb4de2e2868a539846ec48dd43fd623c2ba69a5. However the failure was still observed on select machines. This change addresses the remaining failures by ensuring that any new WatchKey is held until the polling thread becomes active.

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Commit messages:
 - Add Thread.sleep to PollingWatchService's register method

Changes: https://git.openjdk.org/jdk/pull/10824/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10824&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297275
  Stats: 9 lines in 1 file changed: 8 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/10824.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10824/head:pull/10824

PR: https://git.openjdk.org/jdk/pull/10824