[code-reflection] Integrated: Use debug system property

[Paul Sandoz]

Use a system property for the debug value. When set also output the Java code model that is transformed to an ONNX code model. Tweak the example code+comments.

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Commit messages:
 - Use debug system property.

Changes: https://git.openjdk.org/babylon/pull/345/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=345&range=00
  Stats: 53 lines in 7 files changed: 24 ins; 13 del; 16 mod
  Patch: https://git.openjdk.org/babylon/pull/345.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/345/head:pull/345

PR: https://git.openjdk.org/babylon/pull/345