Add randomized MAC (last used)

Makes it clear that when a network is disconnected, the randomized MAC
address shown is the last used one. (ie. it could change in the next
connection depending on whether enhanced MAC randomization is active)

Bug: 170166681
Test: Manually verified that the new text is shown for disconnected
networks.

Change-Id: I4aec47b2de68c077e6fdc7efa96d39e6666d4c86