services/device/devicex: fix group name handling on darwin
services/device/devicex: fix group name handling on darwin

Darwin's account creation conventions differ from Linux: no group is
automatically made for each account. Consequently, multiuser device
manager installs must not try to use the device manager account name
as a group name.

Change-Id: I8414db060f12a6fe1470314509fdd9c165fce05a

vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref