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

Are you sure this was in MediaBrowserActivity.java?
* https://github.com/wordpress-mobile/WordPress-Android/blob/issue/1068-ugly-media-layout-tablet/WordPress/src/main/java/org/wordpress/android/ui/media/MediaBrowserActivity.java#L539
* https://github.com/wordpress-mobile/WordPress-Android/blob/issue/1068-ugly-media-layout-tablet/WordPress/src/main/java/org/wordpress/android/ui/media/MediaBrowserActivity.java#L130

I agree it's useless to have a List or Set here since they will only be ArrayList or HashSet but still valid.