TIF: Use XML file for representing TvContentRating

This change addresses the following API council feedback.

- Remove all constants from TvContentRating. Instead this class becomes a
  general parser for a tuple of "rating domain"+type+subtype(s).
- Instead of constants in the API, the rating definitions should come from
  parsed XML meta-data tied to a TvInputService entry in the AndroidManifest.
  This XML should define a ranked order of the rating constants along with
  @string references for displaying in the Settings UI.  Mention that the
  "rating domain" should be scoped similar to a package name, for example
  "com.youtube.ratings".
- For system-defined rating types (like US, Korea, etc) we should parse this
  same XML format, but it may come from a hard-coded XML resource (since they
  aren't tied to a single TvInputService.)
- Thoroughly document the built-in supported types in javadoc on TvContentRating.

Change-Id: I0a9526c73c8ca67fd0eeac63f3c63c05657a45e3