I want to retain a bitmap when changing orientation of screen

I made a custom ImageView which has a canvas and gets two bitmaps so that I can draw on top of an existing image. While the orientation changes the images is visible but the lines I've drawn disappear.

public class DrawableImageView extends ImageView implements View.OnTouchListener, DrawableImageViewControlListener {
float downx = 0;
float downy = 0;
float upx = 0;
float upy = 0;

Canvas canvas;
Paint paint;
Matrix matrix;

boolean isChanged;

public DrawableImageView(Context context) {
    super(context);
    setOnTouchListener(this);
}

public DrawableImageView(Context context, @Nullable AttributeSet attrs) {
    super(context, attrs);
    setOnTouchListener(this);
}

public DrawableImageView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
    super(context, attrs, defStyleAttr);
    setOnTouchListener(this);
}

public void setNewImage(Bitmap alteredBitmap, Bitmap bmp)
{
    canvas = new Canvas(alteredBitmap);
    paint = new Paint();
    paint.setColor(Color.RED);
    paint.setStrokeWidth(5);
    paint.setStrokeCap(Paint.Cap.ROUND);
    matrix = new Matrix();
    canvas.drawBitmap(bmp, matrix, paint);
    isChanged = false;
    setImageBitmap(alteredBitmap);
}


@Override
public boolean onTouch(View v, MotionEvent event) {
    int action = event.getAction();
    isChanged = true;

    switch (action)
    {
        case MotionEvent.ACTION_DOWN:
            downx = getPointerCoords(event)[0];//event.getX();
            downy = getPointerCoords(event)[1];//event.getY();
            break;
        case MotionEvent.ACTION_MOVE:
            upx = getPointerCoords(event)[0];//event.getX();
            upy = getPointerCoords(event)[1];//event.getY();
            canvas.drawLine(downx, downy, upx, upy, paint);
            invalidate();
            downx = upx;
            downy = upy;
            break;
        case MotionEvent.ACTION_UP:
            upx = getPointerCoords(event)[0];//event.getX();
            upy = getPointerCoords(event)[1];//event.getY();
            canvas.drawLine(downx, downy, upx, upy, paint);
            invalidate();
            break;
        case MotionEvent.ACTION_CANCEL:
            break;
        default:
            break;
    }
    return true;
}

final float[] getPointerCoords(MotionEvent e)
{
    final int index = e.getActionIndex();
    final float[] coords = new float[] { e.getX(index), e.getY(index) };
    Matrix matrix = new Matrix();
    getImageMatrix().invert(matrix);
    matrix.postTranslate(getScrollX(), getScrollY());
    matrix.mapPoints(coords);
    return coords;
}

@Override
public void colorChanged(int color) {
    paint.setColor(color);
}

@Override
public void brushChanged(int size) {
    paint.setStrokeWidth(size);
}

public boolean isChanged() {
    return isChanged;
}

And here is the code where I initialize the ImageView in my OnCreate:

imageView = (DrawableImageView) findViewById(R.id.editImageView);
        IStorage storage = BootLoader.resolve(this).getStorage();
        imageFile = storage.getImageFile(fileName);
        Bitmap bitmap = BitmapFactory.decodeFile(imageFile.getAbsolutePath());
        if (alteredBitmap == null) {
            alteredBitmap = Bitmap.createBitmap(bitmap.getWidth(), bitmap.getHeight(), bitmap.getConfig());
        }
        imageView.setNewImage(alteredBitmap, bitmap);

Note: I am using two layout files for the two different orientations, and would like to keep it this way. So not manually handling configuration changes.

Android tears down the whole Activity when the screen orientation is changed and restarts it again, so you need to save the altered bitmap when this happens and restore it afterwards.

Different storage options are described here

When to save is described here,