Make ngeoDecorateInteraction use getActive/setActive

OpenLayers interactions now have an `active` property, plus `getActive` and `setActive` accessors. This PR changes the `ngeoDecorateInteraction` service to use `getActive` and `setActive` rather than testing whether the map has the interaction, and activating/deactivating the interaction by adding it to/removing it from the map.

Please review.