APKs can now use XML files for vector graphics like the app icon. `aapt`
returns the XML file by default, and perhaps also androguard. This
checks if the icon is an XML file, and if so, it tries to find a PNG in
the APK with the same name and density to use instead
closes#322
This should also ultimately make the XML file available as an icon source
as well fdroidclient#1091