Bug 950070 - Perform correct localization of the roaming notifier on lockscreen

The localize() method takes as a third argument an object, not a string.
Doing this like this results in a lockscreen not substituting correctly
and hence displaying ``{{operator}} (Roaming)'' to the user.
Merge pull request #14658 from lissyx/bug950070

Bug 950070 - Perform correct localization of the roaming notifier on lockscreen r=kaze