ICU-22901 Update Locale::init() to use StringPiece

[roubert]

Checklist

• Required: Issue filed: ICU-22901
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• 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

[roubert]

This replaces the last instance in the locale code where a copy of some string is being made only because it needs to be NUL terminated for some internal API.