Display "Unavailable" for default MAC

Display "Unavailable" 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 RunSettingsLibRoboTests -j40)
Change-Id: I2adab477a43262dd0067bc30b0a9ca4d35dc4298
2 files changed