If we have an app available from more than one remote, and also have
an installed app with * origin in the plugin cache, we end up always
returning the same instance when doing the plugin cache lookup. This
leads to the source selection drop down in the details page not working
as we never get any alternates, always just the same installed instance
back from the plugin cache.

This commit fixes this by making sure the origin part of the unique ID
is set when we add installed apps to the plugin cache.

GNOME/gnome-software#557