Handle always present wired ethernet interfaces at boot.

Devices with always present wired ethernet interfaces (as opposed
to usb ethernet dongles) don't have the interface added and removed.
Instead, it is present already at boot time.  Change EthernetDataTracker
to support the already existing at boot time case.

Change-Id: I7b8e938f6b07aabd14f00ace55b35b7a05ea5a38
Signed-off-by: Mike J. Chen <mjchen@google.com>