Update disableAutoFocus code to only interfere once it knows it should

In essence, this changes behaviour from
 - blur as soon as an input is focused
 - refocus the last focused input if Vimium is disabled or
   disableAutoFocus = false, and stop listening for new 'focus' events
to
 - do nothing if we're waiting to find out whether Vimium is enabled and
   disableAutoFocus = true
 - blur the last focused element when we know the conditions are true
 - blur all future focused elements as soon as they are focused.

 This is more in line with the default value disableAutoFocus = false,
 since we're not acting unless the user wants us to.