have fallback for XML icons also consider res/mipmap*

have fallback for XML icons also consider res/mipmap*

several apps (e.g.
[FastHub](http://apt.qumran.org/fdroid/index/apk/com.fastaccess.github)
and [Monety](http://apt.qumran.org/fdroid/index/apk/open.currency)) have
their ic_launcher.png files not in res/drawable*dpi/, 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").