Add xxhdpi icons
Use app-specific directory on the SD card

Starting with KitKat, this is the official way to store files on external
storage. Actually, Google says that previous ways to do so were not part
of the API.

Here is an article about this: http://www.androidpolice.com/?p=229574

Old files stored  on the root of the SD card will be cleaned up, but only
once (unless the user decides to clean data of the installer).