Android : Palette in ListView Android

I have a ListView and I'm playing with Palette from the support library. When on using BitmapFactory.decodeStream to generate a bitmap from a url, this throws an exception (Network on UI thread) or possibly very expensive. How do I make this asynchronous? I couldn't think of any efficient way to do this. What's the best approach?

    @Override
    public View getView(int position, View convertView, ViewGroup viewGroup) {
        final ViewHolder holder;
        if (convertView == null) {
            convertView = mInflater.inflate(R.layout.item_grid, null);
            holder = new ViewHolder();
            holder.image = (ImageView) convertView.findViewById(R.id.img_photo);
            holder.bg = (LinearLayout) convertView.findViewById(R.id.bg_title);
            holder.text = (TextView) convertView.findViewById(R.id.txt_title);
            convertView.setTag(holder);
        } else {
            holder = (ViewHolder) convertView.getTag();
        }

        Picasso.with(mContext)
                .load(mShows.get(position).poster)
                .into(holder.image);

        try {
            URL url = new URL(mShows.get(position).poster);
            Bitmap bitmap = BitmapFactory.decodeStream(
                    url.openConnection().getInputStream()); // Too expensive!!!
            Palette palette = Palette.generate(bitmap);

            holder.text.setText(mShows.get(position).title);
            holder.text.setTextColor(palette.getVibrantColor().getRgb());

            holder.bg.setAlpha(0.4f);
            holder.bg.setBackgroundColor(palette.getDarkMutedColor().getRgb());

        } catch (IOException e) {
            e.printStackTrace();
        }

        return convertView;
    }