How to create a ‘transparent circle inside rectangle’ shape in XML in Android?

I’ve been playing recently with something similar, and adapted it for you.
All the magic is happening in the onDraw :

public class FocusView extends View {
  private Paint mTransparentPaint;
  private Paint mSemiBlackPaint;
  private Path mPath = new Path();

  public FocusView(Context context) {
    super(context);
    initPaints();
  }

  public FocusView(Context context, AttributeSet attrs) {
    super(context, attrs);
    initPaints();
  }

  public FocusView(Context context, AttributeSet attrs, int defStyleAttr) {
    super(context, attrs, defStyleAttr);
    initPaints();
  }

  private void initPaints() {
    mTransparentPaint = new Paint();
    mTransparentPaint.setColor(Color.TRANSPARENT);
    mTransparentPaint.setStrokeWidth(10);

    mSemiBlackPaint = new Paint();
    mSemiBlackPaint.setColor(Color.TRANSPARENT);
    mSemiBlackPaint.setStrokeWidth(10);
  }

  @Override
  protected void onDraw(Canvas canvas) {
    super.onDraw(canvas);

    mPath.reset();

    mPath.addCircle(canvas.getWidth() / 2, canvas.getHeight() / 2, 550, Path.Direction.CW);
    mPath.setFillType(Path.FillType.INVERSE_EVEN_ODD);

    canvas.drawCircle(canvas.getWidth() / 2, canvas.getHeight() / 2, 550, mTransparentPaint);

    canvas.drawPath(mPath, mSemiBlackPaint);
    canvas.clipPath(mPath);
    canvas.drawColor(Color.parseColor("#A6000000"));
  }
 }

The trick here is to create a Path (the transparent circle) so that we can set the drawing method of the path to be “outside of the path” instead of “inside of the path”. Finally we can simply clip the canvas to that path, and fill in the black color.

For you, you’ll just need to change Color.BLACK to your color, as well as change the desired radius.

EDIT :
Oh and simply add it programmatically :
FocusView view = new FocusView(context)
your_layout.addView(view)

Or by XML :

<package_path_to_.FocusView
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

EDIT2 : I just saw you wanted this for the onboarding of your app.
You might consider having a look at https://github.com/iammert/MaterialIntroView then