libebook: Export the API for phone region guessing

libebook: Export the API for phone region guessing

This adds e_phone_number_get_country_code_for_region() and
e_phone_number_get_default_region() which give access to the
region guessing mechanisms. They are needed to efficently
build the sqlite backend's phone number indexes.

See: https://bugzilla.gnome.org/show_bug.cgi?id=689622