Package organization

refactoring to make the sample application easier to understand by having a more detailed package structure

leanback/data
leanback/model
leanback/presenter
leanback/recommendation
leanback/ui

The branch also has the following small refactorings which help starters understand the app:

- moved class GridItemPresenter up to be a top level class (was an inner class of MainFragment)
- renamed DetailsActivity to MovieDetailsActivity and LeanbackDetailsFragment to MovieDetailsFragment to have a consistent naming convention of activities and fragments