Do not decompress icons when first running gnome-software

Other AppStream implementations do not support loading from a tarball cache, and
given packages and LiveCDs are both compressed there is little point doing it.

This also allows gnome-software to load icons installed as part of other
AppStream metadata providers.
Do not include the Fedora AppStream data in the upstream tarball
Ubuntu installs AppStream data into /var/cache/app-info/xmls so support that too