[jdk25u] RFR: 8372753: jpackage ignres --file-associations option with predefined app image

[Alexey Semenyuk]

Fix handling of the `--file-associations` option when bundling from the predefined app image. Add relevant tests.

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Commit messages:
 - Add tests
 - Fix the implementation

Changes: https://git.openjdk.org/jdk25u/pull/395/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk25u&pr=395&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372753
  Stats: 44 lines in 3 files changed: 37 ins; 3 del; 4 mod
  Patch: https://git.openjdk.org/jdk25u/pull/395.diff
  Fetch: git fetch https://git.openjdk.org/jdk25u.git pull/395/head:pull/395

PR: https://git.openjdk.org/jdk25u/pull/395