RFR: 8299772: The ColorModel.getRGBdefault() method is not thread-safe

[Sergey Bylokhov]

The getRGBdefault() was made thread safe via holder implemented as a local interface.

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Commit messages:
 - Update RGBdefaultSingleton.java
 - the test
 - 8299772: The ColorModel.getRGBdefault() method is not thread-safe

Changes: https://git.openjdk.org/jdk/pull/11895/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11895&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299772
  Stats: 80 lines in 2 files changed: 70 ins; 3 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/11895.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11895/head:pull/11895

PR: https://git.openjdk.org/jdk/pull/11895