RFR: 8278609: accessibility frame is misplaced on a secondary monitor on macOS

[Aleksandr Veselov]

Use primary screen instead of window main screen to properly convert mouse position from screen coordinates to window ones.

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Commit messages:
 - Replace [] syntax with `objectAtIndex` call
 - 8278609: accessibility frame is misplaced on a secondary monitor on macOS

Changes: https://git.openjdk.java.net/jdk/pull/6815/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6815&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278609
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6815.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6815/head:pull/6815

PR: https://git.openjdk.java.net/jdk/pull/6815