download image from viewpager in which image is stored in raw file

Question

I've made a image slider from viewPager. The image shown in viewPager is stored in raw folder inside the project of android studio.

I want to request for the permission of user to download image, display progress bar when image is downloading and finally a tost message when image is downloaded.

Things I've done:
-I have made a imageview button to download on the viewpager.
-Progressbar with android:visibility="invisible"
-My code in MainActivty where image and download option are shown.

public class MainActivity extends AppCompatActivity {
    ViewPager viewPager;
    ProgressBar progressBar;
    private int[] imageUrls = new int[]{
            R.drawable.after_cookie,
            R.drawable.before_cookie,
            R.drawable.androidparty
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        progressBar = findViewById(R.id.progress);
        viewPager = findViewById(R.id.view_pager);
        final ViewPageAdapter adapter = new ViewPageAdapter(this, imageUrls);
        viewPager.setAdapter(adapter);

        ImageView btnDownload = findViewById(R.id.btnDownload);
        btnDownload.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                //what should I do? please help :(
            }
        });
    }
}

code of viewpager adapter:

public class ViewPageAdapter extends PagerAdapter {

    private Context context;
    private int[] imageUrls;
    public View currentImageView;

    ViewPageAdapter(Context context, int[] imageUrls) {
        this.context = context;
        this.imageUrls = imageUrls;
    }

    @Override
    public int getCount() {
        return imageUrls.length;
    }

    @Override
    public boolean isViewFromObject(@NonNull View view, @NonNull Object object) {
        return view == object;
    }

    @NonNull
    @Override
    public Object instantiateItem(@NonNull ViewGroup container, int position) {
        ImageView view = new ImageView(context);

        Picasso.get()
                .load(imageUrls[position])
                .into(view);
        container.addView(view);
        return view;
    }

    @Override
    public void destroyItem(@NonNull ViewGroup container, int position, @NonNull Object object) {
        container.removeView((View) object);
    }

    @Override
    public void setPrimaryItem(ViewGroup container, int position, Object object) {
        currentImageView = (View)object;
    }
}