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
2 jobs for some-data-related-fixed in 36 minutes and 43 seconds (queued for 12 seconds)
Status Job ID Name Coverage
  Test
passed #21311397
metadata_v0

00:03:48

passed #21311394
test

00:36:43