RFR: 8353196: [macos] Contents of ".jpackage.xml" file are wrong when building .pkg from unsigned app image

[Alexey Semenyuk]

- Don't create ".jpackage.xml" file in image for DNG or PKG bundles;
 - Remove ".jpackage.xml" file from unsigned predefined app image for DNG or PKG bundles;
 - Set the value of `signed` property in ".jpackage.xml" file to "true" only in the app image itself is signed.

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Commit messages:
 - Fix JPackageCommand.macAppImageSigned() and move it together with JPackageCommand.signPredefinedAppImage() to MacHelper class
 - Strip ".jpackage.xml" from copied app image when permitted
 - Don't create app image file on macOS when not needed
 - Adjust JPackageCommand.assertAppImageFile()

Changes: https://git.openjdk.org/jdk/pull/24313/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24313&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8353196
  Stats: 83 lines in 5 files changed: 66 ins; 6 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/24313.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24313/head:pull/24313

PR: https://git.openjdk.org/jdk/pull/24313