Bug 880390: Only add 'callschanged' listener once.

On b2g18 after bug 823958, adding an event listener can trigger another
event.  Since the callschanged listener is added from an event, this
can cause an infinite loop.

The simplest solution, recommended by :bent, is to avoid adding the listener
if its already registered.
Merge pull request #10294 from wanderview/system-statusbar-callschanged

Bug 880390: Only add 'callschanged' listener once. r=etienne