Factory for creating new tracker instances. As new items are detected, each distinct item (as indicated by an item ID generated by the detector) will have a new instance of the tracker created for that item. A tracker will receive all notifications for that item for consecutive frames, until the item has been determined to be no longer present.

When onDone() is called on a tracker instance, no future notifications will be sent to that instance. Associated resources, if any, should be freed upon receiving that call.