overhaul the app icon delivery and display

Starting with android-21, it is possible to use vector graphics files (e.g. ic_launcher.xml) for the app icon. With android-26, Google is further pushing that format with its "adaptive icons". fdroidclient should support the XML files as the first choice of icons to download.

This is then also a good opportunity to overhaul App.icon, App.iconUrl, and App.iconUrlLarge. These are inconsistently available and used. Then there is the 512x512 icon.png that in the localized block that comes from the app's source repo. I think we should first try to use icon.png if its available for the current locale (unless that's the default en-US). Otherwise, it should try to use the vector XML icon, then last resort, use the old per-DPI PNG icons.

This is related to fdroidserver#322 (closed)