These refer to distinct resource sets. The suffixes (e.g., -mdpi, -hdpi) are filters,
indicating under what circumstances the images stored in those directories should
be used. Specifically, -ldpi indicates images that should be used on devices with
low-density screens (around 120 dots-per-inch, or “dpi”). The -mdpi suffix indicates
resources for medium-density screens (around 160dpi), -hdpi indicates resources for
high-density screens (around 240dpi)