code: public static TileCache createTileCache(Context c, String id, int tileSize, float screenRatio, double overdraw) {
          return createTileCache(c, id, tileSize, screenRatio, overdraw, false);
      }
docs: Utility function to create a two-level tile cache with the right size. When the cache is created we do not
  actually know the size of the mapview, so the screenRatio is an approximation of the required size. This is the
  compatibility version that by default creates a non-persistent cache.
  
  @param c           the Android context
  @param id          name for the storage directory
  @param tileSize    tile size
  @param screenRatio part of the screen the view takes up
  @param overdraw    overdraw allowance
  @return a new cache created on the external storage