Use flatpak_installation_list_installed_refs for listing installed flatpaks

Summary:
Instead of going through a directory. This allows us to have the
FlatpakInstalledRef* tuple for each of the resources being able to fully
populate the resources with architecture, branch and origin.

This is also useful because since we have all of the information we get
to use these tuples to generate a uniqueId so they can be lookedup by
comparing it. Therefore, we remove the fallbacks on getInstalledRefForApp
and getAppForInstalledRef.

Reviewers: jgrulich

Reviewed By: jgrulich

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D7796