[foreign-jextract] RFR: updating samples. changed constants class to use package constructor

[Athijegannathan Sundararajan]

fixed to use package private constructors for constants classes. piggybacking to
include latest samples.

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Commit messages:
 - updating samples. changed constants class to use package constructor

Changes: https://git.openjdk.java.net/panama-foreign/pull/292/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/292/webrev.00
  Stats: 247 lines in 3 files changed: 246 ins; 1 del; 0 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/292.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/292/head:pull/292

PR: https://git.openjdk.java.net/panama-foreign/pull/292