Traffic Plugin add above road layer

[tobrun]

We added a Traffic Plugin in #6, currently we hardcode the layerId which we use in addLayerBefore(layer, layedId). Ideally we use can use the source layer identifier to check for a road identifier instead.

Upstream feature ticket for source layer identifier in mapbox/mapbox-gl-native#8663

[tobrun]

Adding here that we need to use source layer identifier in setVisibility as well.