Add country mapping metadata support

In tooling that generates tzlookup.xml, add support for metadata
that excludes selected country -> time zone mappings
from use by a user-facing time zone picker.

This change just adds support to the tzlookup.xml generation.
Later changes will update the data files and tzlookup.xml parsing
logic.

Bug: 72144448
Test: Manual testing - ran update-tzdata.py
Change-Id: I0be9e1adcf75972d42848c26629d9f71de5d329b