<AWT Dev> RFR: 8267066: New NSAccessibility peers should return they roles and subroles directly

[Alexander Zuev]

Made all implementing classes return their roles directly.
Change nullability attributes to the singular style across new classes.

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Commit messages:
 - 8267066: New NSAccessibility peers should return they roles and subroles directly

Changes: https://git.openjdk.java.net/jdk/pull/4162/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4162&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8267066
  Stats: 76 lines in 25 files changed: 42 ins; 0 del; 34 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4162.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4162/head:pull/4162

PR: https://git.openjdk.java.net/jdk/pull/4162