Added a class that represents TV content ratings that are used for parental controls. TV input services are required to create TvContentRating objects with rating/sub-rating constants for specific countries (for now US and KR only) defined in this class and then convert them into strings in order to store them in the EPG database. Bug: 13172379 Change-Id: I6200cb12fbc274dd0a8bcb87a8ac3cd2d96791db