fix #1068 ugly media layout tablet

fix #1068 ugly media tablet layout

* Removed specific tablet layout
* Removed MultiSelectGridView (now use stock GridView)
* Cleanup / moved code from MediaBrowserActivity to the corresponding Fragment and Adapter
* Fixed grid spacing / margins
* Fixed untranslatable strings

This can be a single "%d selected" string, as shown [here](http://developer.android.com/design/patterns/selection.html)