RFR: 8378578: Add final to XbmColormap and XbmHints in XbmImageDecoder

[Alexey Ivanov]

A trivial clean up for `XbmImageDecoder` that marks the fields `XbmColormap` and `XbmHints` with the `final` modifier. These fields are constants.

In addition to the above, improve the javadoc by using the `@snippet`. Otherwise, all the lines of the XBM snippet are combined into one line.

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Commit messages:
 - 8378578: Add final to XbmColormap and XbmHints in XbmImageDecoder

Changes: https://git.openjdk.org/jdk/pull/29896/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29896&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8378578
  Stats: 9 lines in 1 file changed: 3 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/29896.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29896/head:pull/29896

PR: https://git.openjdk.org/jdk/pull/29896