Show the status of private DNS.

This works as follows :
Off → "Off"
Opportunistic, inactive → "Automatic"
Opportunistic, active → "On"
  (stealing a string from notifications for this)
Strict, not resolved and/or not validated → "Couldn't connect"
Strict, resolved and validated → Set up hostname

Bug: 73641539
Test: manual, and updated tests pass
Change-Id: Id1132467288d51aa9cb81a04db65dee438ddfad9