Resize image to xxhdpi xhdpi hdpi mdpi.
The original image is designed in ndp * 18 px. For example: an icon(48dp * 48dp) will be designed in 864px * 864px. Then the script will generate images in xxhdpi xhdpi hdpi and mdpi.
resize_img_ratio6.py is similar, but the original image is designed in ndp * 6 px.