ICU-22105 Fixed the unit-conversion logic to work correctly with nega…

[richgillam]

…tive temperature values.

Checklist

• [x] Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22105
• [x] Required: The PR title must be prefixed with a JIRA Issue number.
• [x] Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• [x] Required: Each commit message must be prefixed with a JIRA Issue number.
• [x] Issue accepted (done by Technical Committee after discussion)
• [x] Tests included, if applicable
• [ ] API docs and/or User Guide docs changed or added, if applicable