API updates for MediaRouter

* Accept a Context when fetching the names of routes and
  categories. This lets string resources resolve at time of access
  with the correct configuration. The older versions remain available
  that will use the static resources from the application. (There are
  enough cases where applications will populate this from external
  data that requiring it each time even when it was not initialized
  from a resource doesn't seem reasonable.)

* Remove the ability for apps to programmatically select non-user
  routes.

* Make MediaRouter.Callback an abstract class instead of an interface.
  This will make further extensions easier to keep compatible in the
  future.

Change-Id: If981c511dfbdfaf41ef0d1cfe4a377fc14bb5600