Add Dbus.Service property to modem devices.

Add DBus.Service property to modem devices so that Chrome can watch
for signals on the service name rather than the connection name.

BUG=chromium-os:31575
TEST=Read instructions in fake-gsm-modem for running with flimflam.
TEST=run /usr/local/lib/flimflam/test/fake-gsm-modem -s 3 -S, ctrl-c, re-run

Change-Id: Ide5d0a9250ff9ca176805a22e2e077b28ad221ae
Reviewed-on: https://gerrit.chromium.org/gerrit/25314
Tested-by: Jason Glasgow <jglasgow@chromium.org>
Reviewed-by: Ben Chan <benchan@chromium.org>