RFR: 8298645 JNI works with accessibleSelection on a wrong thread

[Artem Semenov]

[ComboBoxAccessibility accessibilityValue] works with the accessibleSelection object directly, however the work should go through CAccessibility so that it is executed on the Event Dispatch thread.

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Commit messages:
 - 8298645 JNI works with accessibleSelection on a wrong thread

Changes: https://git.openjdk.org/jdk/pull/11653/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11653&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298645
  Stats: 53 lines in 2 files changed: 20 ins; 20 del; 13 mod
  Patch: https://git.openjdk.org/jdk/pull/11653.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11653/head:pull/11653

PR: https://git.openjdk.org/jdk/pull/11653