RFR: 6434110: Color constructor parameter name is misleading

[Sergey Bylokhov]

Small cleanup: the parameter names in the Color(int, boolean) constructor have been renamed:
 - rgba -> argb, since this is the format described in the spec
 - hasalpha -> hasAlpha, to follow common naming conventions

Also adds a basic test for the constructor.

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Commit messages:
 - 6434110: Color constructor parameter name is misleading

Changes: https://git.openjdk.org/jdk/pull/29734/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29734&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-6434110
  Stats: 78 lines in 2 files changed: 66 ins; 0 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/29734.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29734/head:pull/29734

PR: https://git.openjdk.org/jdk/pull/29734