Clean up MozL10n API use in Cost Control #1057795

This is relating bug 1057795
please, pay attention to changes you are making. You change the variable reference to a node (calltimeSpan) to another here. And then you try to assign to its textContent a result of a method setAttributes that does not return any value.

Indended code should look like:

navigator.mozL10n.setAttributes(calltimeSpan, 'magnitude', {
  value: Formatting.computeTelephonyMinutes(activity),
  unit: 'min.'
});