have fallback for XML icons also consider res/mipmap*

several apps (e.g. FastHub and Monety) have their ic_launcher.png files not in res/drawabledpi/,* but in res/mipmap/* -- so the regex has been adjusted by this patch.

Additionally: if the only icon for a given resolution was an XML without existing fallback-PNG, it should be considered "non existent" (ie. "empty_density").