MediaBrowser: Make ItemCallback can take a null media item

When the implementation of MediaBrowserService returns null,
ItemCallback.onError was called, but the JavaDoc is saying that
ItemCallback.onItemLoaded can be called with a null result.

This CL makes ItemCallback.onItemLoaded be able to take a null result,
and ItemCallback.onError be called when the implementation doesn't override
the onLoadItem method or when the returned object is not MediaItem.

Bug: 27808084
Change-Id: I97cfd83786f8f857dc9551e5b0a358962f98e4a2