audio: Fix event handler's argument name

Every emit of this event caused an invisible exception in every audio
listener (e.g. core). The exception was made visible by the change in
the previous commit.

mopidy/audio/listener.py

@@ -37,7 +37,7 @@ class AudioListener(listener.Listener):
         """
         pass
 
-    def position_changed(self, position_changed):
+    def position_changed(self, position):
         """
         Called whenever the position of the stream changes.