Fix positionInterval clear

[ramybenaroya]

The positionInterval is created in https://github.com/InteractiveAdvertisingBureau/openvv/blob/master/src/org/openvv/js/OVVAsset.js#L1398. In runtime the context of createBeacons is an instance of OVVAsset. Currently the interval is never cleared because window.$ovv.positionInterval is never set and is undefined.