service-client: Fix a check for system app

We don't want to stop a system app client on agent's behalf but our check
for system apps was inverted so we ended up doing the opposite. :( Thanks
to Florian Müllner for pointing this out.
3 jobs for wip/fix-system-app-check in 12 minutes and 55 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #37975
ubuntu-16.04

00:08:23

passed #37976
ubuntu-17.10

00:05:35

passed #37977
ubuntu-18.04

00:05:20