Convert old-style Favorites resources in KActivities DB

In the past, application Favorites were stored with their .desktop file path
such as "/usr/share/applications/org.kde.dolphin.desktop". But now those
entries are stored like so: "applications:org.kde.dolphin.desktop".

This commit converts old DB entries to the new style, which solves the issue
of users with old databases being unable to change their Favorite items.

BUG: 385814
FIXED-IN: 5.24.5


(cherry picked from commit ff446e23)