Don't display an invalid Wi-Fi MAC address

Only display the WifiInfo MAC address if it is a valid address.
Otherwise, display "Unavailable".  The underlying WifiInfo class
now has a default value, and a method that can be used to test
whether or not the contained address is valid.  This change uses
this test and avoids having the confusing default address displayed.

Bug: 32478606
Test: Manual: Turn off WiFi and reboot -- make sure "Unavailable"
      is displayed as the Wi-Fi MAC Address as opposed to "02:00:00:00:00:00"

Change-Id: I912804eb65735375e0ca3c4618a6399543f33b57