Display "MAC address is randomized" in Wifi Preference Page

When Connected MAC Randomization is enabled and the device is
disconnected, there isn't a valid MAC address to display since we don't
know what the device's next MAC address is going to be.

Instead of displaying the MAC address used for previous connection, tell
users that "MAC address is randomized."

Bug: 73663275
Test: make -j40 RunSettingsRoboTests
Change-Id: Id5352926f221e3215734d1c198f2072940f82695