RFR: 8276186: Require getAvailableLocales() methods to include Locale.ROOT

[Naoto Sato]

This fix is to require to include `Locale.ROOT` in the returned arrays/set from `getAvailableLocales()` methods in various locale-sensitive classes. The implementation has been including `Locale.ROOT` since its inception, it is simply a doc clarification (+ a test case verifying it). Corresponding CSR has also been drafted: https://bugs.openjdk.java.net/browse/JDK-8276249

-------------

Commit messages:
 - Minor fixup
 - Reflected CSR modifications
 - Added a test case
 - 8276186: Include Locale.ROOT in getAvailableLocales() methods

Changes: https://git.openjdk.java.net/jdk/pull/6258/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6258&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276186
  Stats: 108 lines in 10 files changed: 86 ins; 0 del; 22 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6258.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6258/head:pull/6258

PR: https://git.openjdk.java.net/jdk/pull/6258