update: find PNG when recommended icon is an XML file
update: find PNG when recommended icon is an XML file

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

fdroidtravis/fdroidserver,f-droid/fdroid-server,f-droid/fdroidserver,f-droid/fdroidserver,f-droid/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroid-server,f-droid/fdroidserver,fdroidtravis/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroidserver,f-droid/fdroid-server,f-droid/fdroid-server,f-droid/fdroid-server