Clean up public FontConfig APIs

This CL contains following clean up:

- Hide unnecessary constructors.
- Change List<XX> to XX[] since actually all fields are immutable.
- Change font's variant type from String to int.
- Decouple resource related members to FontResourcesParser.
- Add NonNull/Nullable to all fields.

Test: ran android.content.res.FontResourcesParserTest

Change-Id: If456266ffff86d41342572a19662cc8f3cd13181