TIF: Enable applications to register custom content rating systems

A recent change that moved the string resource for the system supported
content ratings from the framework to the TV app led to a need for
allowing the TV app to publish its own content rating systems. This
change added an intent action and a metadata key to be used by the TV
input manager service for querying available content rating systems,
similarly to the way InputManager defined ACTION_QUERY_KEYBOARD_LAYOUTS
and META_DATA_KEYBOARD_LAYOUTS for custom keyboard layouts. Applications
now can register their own rating systems simply by declaring a
broadcast receiver in their manifests.

Bug: 17167287, Bug: 17114406
Change-Id: I1808c624014b603ea709714737c2a880714bdc0a