main/postmarketos-ui-{phosh,gnome,plasma}: prepare for using systemd presets

Picked from !5377 (merged). See the description in #2937 (closed) for more info about how systemd presets work to enable services by default. Also note that this will kinda break these UIs w/ systemd at least until the aforementioned MR is merged, but that's fine since the systemd staging branch isn't stable. Having these patches in master is a prerequisite to merging !5377 (merged)

cc @jane400