How to add rectangles on top of existing rectangle in canvas

Try this:

public class RectangleTextView extends View {
    private final Paint mBlackPaint = new Paint();
    private final Paint mRedPaint = new Paint();
    private final TextPaint mTextPaint;

    public RectangleTextView(Context context, AttributeSet attrs) {
        super(context, attrs);
        int valueInDp = (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, 1, getResources().getDisplayMetrics());
        int valueInSp = (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_SP, 20, getResources().getDisplayMetrics());

        mRedPaint.setColor(Color.parseColor("#CC3333"));

        mBlackPaint.setAntiAlias(false);
        mBlackPaint.setColor(Color.BLACK);
        mBlackPaint.setStrokeWidth(valueInDp);
        mBlackPaint.setStyle(Paint.Style.STROKE);

        mTextPaint = new TextPaint(TextPaint.ANTI_ALIAS_FLAG);
        mTextPaint.setColor(Color.BLACK);
        mTextPaint.setTextAlign(Paint.Align.CENTER);
        mTextPaint.setTextSize(valueInSp);

        mWindowPaint = new Paint();
        mWindowPaint.setColor(Color.parseColor("#CC3333"));
        mWindowPaint.setStrokeWidth(valueInDp);
    }

    private Paint mWindowPaint;

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        if (getWidth() == 0)
            return;

        //initialise red rectangles
        int h = canvas.getHeight();


        int topRectHeight = getPaddingTop();
        int bottomRectHeight = getPaddingBottom();


        //draw end rectangles
        int mSideRectWidth = 10;
        canvas.drawRect(0, 0, mSideRectWidth, getHeight(), mRedPaint); //draw left end rectangle
        canvas.drawRect(getWidth() - mSideRectWidth, 0, getWidth(), getHeight(), mRedPaint); //draw right end rectangle

        //draw grey boxes
        setBackgroundColor(Color.parseColor("#808080"));
        int boxWidth = (getWidth() - mSideRectWidth) / 7;


        int redRectWidth = boxWidth / 5;
        int redRectSpace = redRectWidth / 3;

        //draw text views
        for (int i = 0; i < 7; i++) {
            canvas.drawText(Integer.toString(i + 1), (i * boxWidth + 10) + (boxWidth / 2), ((canvas.getHeight() / 2) - ((mTextPaint.descent() + mTextPaint.ascent()) / 2)), mTextPaint);

            int baseStartX = i * boxWidth;

            //draw red windows
            for (int j = 0; j < 4; j++) {
                mWindowPaint.setStyle(Paint.Style.STROKE);//add this
                int left = mSideRectWidth + baseStartX + (j * (redRectWidth + redRectSpace));
                int right = left + redRectWidth;
                if (j == 1) {
                    mWindowPaint.setStyle(Paint.Style.FILL); // change to this
                }

                Rect rect = new Rect(left, 0, right, topRectHeight);
                canvas.drawRect(rect, mWindowPaint);
                Rect rect2 = new Rect(left, h - bottomRectHeight, right, h);
                canvas.drawRect(rect2, mWindowPaint);
            }
        }

        //draw black lines
        for (int i = 1; i < 7; i++) {

            int startX = mSideRectWidth + boxWidth * i;
            int startY = 0;
            int stopX = mSideRectWidth + boxWidth * i;
            int stopY = getHeight();
            canvas.drawLine(startX, startY, stopX, stopY, mBlackPaint);

        }
    }
}