Fix query of StartPlasma::hasSystemdService

ListUnitByNames did not quite work as the author (me) expected. Whilst a
search for "adsfasf" yeilded an empty list a well-formed name like
"asdfasdf.service" would return a result to say that service was not
loaded.

This means our runtime detection of xdg-autostart-generator failed.

By using ListUnitFilesByPatterns we can filter on the status in the
query, meaning this code works again. We also can query the case of it
being available but explicitly disabled.

BUG: 433333


(cherry picked from commit 1c7e3064)