RFR: JDK-8231706: Investigate if we can change "Java" dir to "app" dir in jpackage app image

[Alexander Matveev]

Please review the jpackage fix for bug [1] at [2].

This is a fix for the JDK-8200758-branch branch of the open sandbox 
repository (jpackage).

- Changed "Java" dir to "app" dir in jpackage app image on macOS.

[1] https://bugs.openjdk.java.net/browse/JDK-8231706

[2] http://cr.openjdk.java.net/~almatvee/8231706/webrev.00/

Thanks,
Alexander