[jfx21u] RFR: 8329705: Add missing Application thread checks to platform specific a11y methods

[Johan Vos]

8329705: Add missing Application thread checks to platform specific a11y methods

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Commit messages:
 - Backport 0eb4d7196099d817cc6467985b882242845bdd2e

Changes: https://git.openjdk.org/jfx21u/pull/61/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx21u&pr=61&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329705
  Stats: 299 lines in 4 files changed: 299 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx21u/pull/61.diff
  Fetch: git fetch https://git.openjdk.org/jfx21u.git pull/61/head:pull/61

PR: https://git.openjdk.org/jfx21u/pull/61