ICU-23460 Clean up registered collator always in CollationServiceTest

[yumaoka]

In the original code, ICU4J CollationServiceTest may leave a collator registered for testing. When one of these test failed before unregistration, it will affect other test cases. Updated the code with try-finally to ensure a collator registered for testing will be unregistered after the test.

Checklist

• Required: Issue filed: ICU-23460
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-NNNNN Fix xyz"
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• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable
• Approver: Feel free to merge on my behalf

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