[Cuttlefish] Use only a single AppStream ID
Right now Cuttlefish provides both org.kde.plasma.cuttlefish and
org.kde.cuttlefish.desktop AppStream IDs, causing it to appear twice in Discover and
other software center apps.
This patch standardizes on org.kde.plasma.cuttlefish, and provides a compatibility
shim so you can still access it from the old URL (appstream://org.kde.cuttlefish.desktop)
Test Plan: Don't know how to test this
Differential Revision: https://phabricator.kde.org/D23301