Fix device ID selection in the common time service.

Fix an issue I discovered while back-porting this code to master.  The
common time service was using the MAC address of "eth0" (hardcoded) as
its device ID instead of fetching it from the interface it is
currently bound to.  On phones (or any other device with no eth0) this
causes time service to never be able to fetch a device ID as it
should.

Change-Id: Icf8a2006924088efc86065927a648f7f53638657