ICU-22575 Change AvailableFormatsSink to allow locales to inherit ava…

[richgillam]

…ilableFormats items from the root locale.

[Cherry-picking the fix to maint/maint-74 from main]

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22575
• Required: The PR title must be prefixed with a JIRA Issue number.
• 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.
• 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]

@pedberg-icu , @FrankYFTang , @markusicu , @macchiati Looks like this PR is ready to go. Can somebody rubber-stamp this for me? (This is the cherry-pick PR; you've already actually reviewed the code.)