Add a systemd service file for the system mode

Change-Id: If3783b3a86108dd968911613f5f63693cb5161c6
system.pa: Don't use module-udev-detect

We want the system instance to only use the built-in card of VTC-1010.
E.g. USB headsets should be left to the user instances.

Change-Id: Ib558fdaf64fa0780bfae2af7117e58482fd3e0c1