Code changes:
 - Removing all references to possible_number_pattern other than the proto
   buffer itself (and derived files, or hand-crafted files based on it.) This
   information is no longer present in the binary.
 - Refactored metadata loading and removed synchronization for all kinds of
   metadata. Clients may experience quicker loading of alternate formats and
   short number metadata, but no change is required for callers of the library.
 - Reduced visibility of `public` internal API
   `PhoneNumberUtil.createInstance(MetadataSource)` to `private`. MetadataSource
   and all its implementations are non-public so this should not affect public
   usage of the library.