Clean up DoubleLaunchChecker

[jglick]

In #6743 I noticed various nonstandard idioms in this class, which are cleaned up here. Switches the tool to use the regular administrative monitor system.

Testing done

Added test coverage (using jenkinsci/jenkins-test-harness#610). Set a breakpoint to interactively check that the GUI is displayed like other monitors and can be dismissed.

Proposed changelog entries

• Switch the double-launch checker to a regular administrative monitor.

Proposed upgrade guidelines

N/A

Before the changes are marked as ready-for-merge:

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[github-actions]

Please take a moment and address the merge conflicts of your pull request. Thanks!

[Vlatombe]

Just a small suggestion, but looks good.

[timja]

/label ready-for-merge

---

This PR is now ready for merge, after ~24 hours, we will merge it if there's no negative feedback.

Thanks!