Merge "libvulkan: refactor the layer discovery logic"

    std::call_once(once_flag, []() {

        if (driver::OpenHAL()) {

            DiscoverLayers();

            initialized = true;

        }

    });

    {

        static std::mutex layer_lock;

        std::lock_guard<std::mutex> lock(layer_lock);

        DiscoverLayers();

    }

    return initialized;

}