update: make icon extraction less dependent on aapt

update: make icon extraction less dependent on aapt

For androguard, @thezero already developed a way to get all the icons after
only extracting the icon name.  So this uses that for the aapt-based scans
also, to make them less brittle.

This should fix the problem where `fdroid update` was choosing the XML icon
for apps that include one, like NewPipe.

closes fdroid/fdroid-website#192