Add method to retrieve a list of country iso codes

This commit adds a method to retrieve a list of country
ISO codes known to have time zones. Effectively the list
of inhabited, known country codes based on the content
of the tzlookup.xml file.

The intended use for this method is for a new
time zone picker so that it can retrieve Olson IDs via
the methods on the CountryTimeZones object.

The use of the Android tzlookup.xml file instead of
3rd party libs / data directly also means that
Android can easily add extra metadata to tzlookup.xml
in future to support different usecases for the
country list, country -> time zone mapping and time
zone metadata.

Test: vogar libcore/luni/src/test/java/libcore/libcore/util/TimeZoneFinderTest.java
Bug: 72144448
Bug: 72142943
Change-Id: I58fe128c96fd37d54825e77e6a5d4f3822e5e922