Display "Unavailable" when received default MAC

Display "Unavailable" instead of "02:00:00:00:00:00" when the current
MAC address is "02:00:00:00:00:00" which indicates that we couldn't get
the actual device MAC address.

Bug: 110043449
Test: unittest (make RunSettingsRoboTests
ROBOTEST_FILTER=WifiInfoPreferenceControllerTest)
Change-Id: Iac9f81d144fd4c93ac12adaa80e1a55b19a6e186