system-server: Relocate service file under united-service directory

system-server: Relocate service file under united-service directory

The config file of united-service, system-server-sensord.service was
located under packaging with systemd service file, sensord.service.
But sensord.service and system-server-sensord.service are different type
of service files, it is better to spearate them.
To make it clear that system-server-sensord.service is a config file of
united-service, it is relocated under the new directory united-service.

Change-Id: I4678103e7d012135baa892c92094616890a40656
Signed-off-by: SangYoun Kwak <sy.kwak@samsung.com>