ICU-20656 Add icu4j data tracing

[eranl]

Add a dataTracing debug flag, which enables logging of the name of every data or resource file accessed at runtime.

Checklist

• [v] Required: Issue filed: ICU-20656
• [v] Required: The PR title must be prefixed with a JIRA Issue number.
• [v] Required: Each commit message must be prefixed with a JIRA Issue number.
• [v] Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable
• [v] Approver: Feel free to merge on my behalf

[CLAassistant]

All committers have signed the CLA.

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[markusicu]

Could you please restore and fill in the pull request template?

[markusicu]

Is the C++ data tracing documented in the User Guide? If so, then we should document the Java tracing there, too.

[eranl]

Is the C++ data tracing documented in the User Guide?

Yes, here.

If so, then we should document the Java tracing there, too.

Should I do that? How?

[eranl]

Could you please restore and fill in the pull request template?

Done.

[richgillam]

I second Markus's comments, but the actual code changes look fine to me.

[sffc]

Refreshing CI

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Hooray! The files in the branch are the same across the force-push. 😃

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[eranl]

Commits squashed.

[eranl]

Is the C++ data tracing documented in the User Guide?

Yes, here.

If so, then we should document the Java tracing there, too.

Should I do that? How?

What's the process for this?

[sffc]

The file is in the repo; you can edit it in a new PR

https://github.com/unicode-org/icu/blob/main/docs/userguide/icu_data/tracing.md

[eranl]

Done.