ICU-22765 fix uloc_addLikelySubtags on "und@x=private"

[FrankYFTang]

Fix Locale addLikelySubtags when input is "und@x=private"
Since the code are shared by matcher and addLikelySubtags the behavior for this special case need to check an additional condition.
For Java, just add the test case.

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22765
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• Required: Each commit message must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable

[richgillam]

LOKTM...