RFR: 8351067: Enforce Platform::accessibilityActive threading use [v3]

[Andy Goryachev]

> Changed the spec to require access only from the FX application thread.

Andy Goryachev has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:

 - enforce
 - Merge remote-tracking branch 'origin/master' into 8351067.accessibility
 - review comments
 - javadoc

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1728/files
  - new: https://git.openjdk.org/jfx/pull/1728/files/6a1a152e..0f39382c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1728&range=02
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1728&range=01-02

  Stats: 731 lines in 21 files changed: 573 ins; 115 del; 43 mod
  Patch: https://git.openjdk.org/jfx/pull/1728.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1728/head:pull/1728

PR: https://git.openjdk.org/jfx/pull/1728