Replace if/else with evaluator template

[tobrun]

Follow up from #8709 (comment), we should cleanup the Android binding code to avoid if-else checking:

        std::string sourceLayerId;
        if (layer.is<FillLayer>()) {
            sourceLayerId = layer.as<FillLayer>()->getSourceLayer();
        } else if (layer.is<LineLayer>()) {
            sourceLayerId = layer.as<LineLayer>()->getSourceLayer();
        } else if (layer.is<SymbolLayer>()) {
            sourceLayerId = layer.as<SymbolLayer>()->getSourceLayer();
        } else if (layer.is<CircleLayer>()) {
            sourceLayerId = layer.as<CircleLayer>()->getSourceLayer();
        } else {
            mbgl::Log::Warning(mbgl::Event::JNI, "Layer doesn't support source layer");
        }

This can be replaced with the following template:

struct Evaluator {
    std::string operator()(mbgl::style::BackgroundLayer&) { return {}; }
    std::string operator()(mbgl::style::RasterLayer&) { return {}; }
    std::string operator()(mbgl::style::CustomLayer&) { return {}; }

    template <class VectorLayer>
    std::string operator()(VectorLayer& layer) {
        return layer.getSourceLayer();
    }
};
// ...
std::string sourceLayerId = layer.accept(Evaluator());

cc @ivovandongen