Make goog.events.listen return proper listener key

When goog.events.listen receives an array of event types,
it only return null as listener key. So that calling
goog.events.unlistenByKey does not work.

src/ol/mapbrowserevent.js

@@ -199,7 +199,7 @@ ol.MapBrowserEventHandler = function(map) {
       this.handlePointerDown_, false, this);
 
   this.relayedListenerKey_ = goog.events.listen(this.pointerEventHandler_,
-      [ol.pointer.EventType.POINTERMOVE],
+      ol.pointer.EventType.POINTERMOVE,
       this.relayEvent_, false, this);
 
   if (ol.LEGACY_IE_SUPPORT && ol.IS_LEGACY_IE) {