Make KCMShell::authorize() take into account installation status

Right now it only checks authorization, but for our purposes in KCMs, we commonly care about not only that, but also whether or not the KCMs being queried are actually installed. If a KCM isn't installed, it isn't much help to say that it's authorized because the user still can't use it.

BUG: 443984 FIXED-IN: 5.91

cc @aacid who reported the bug and @broulik who wrote this function and @mart who's the maintainer