Improve performance of Bitmap generation

On a virtual android device, QR Code generation is really slow (up to 17 seconds). After further investigation, I found that the method responsible for such slowness is MatrixToImageWriter.toBitmap(...) .

I tried to replace the code with this one adapted from Zxing [here](https://github.com/zxing/zxing/blob/master/android/src/com/google/zxing/client/android/encode/QRCodeEncoder.java):
```java
        int BLACK = 0xFF000000;
        int WHITE = 0xFFFFFFFF;
        int width = bitMatrix.getWidth();
        int height = bitMatrix.getHeight();
        int[] pixels = new int[width * height];

        // All are 0, or black, by default
        for (int y = 0; y < height; y++) {
            int offset = y * width;
            for (int x = 0; x < width; x++) {
                pixels[offset + x] = bitMatrix.get(x, y) ? BLACK : WHITE;
            }
        }

        Bitmap bitmap = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888);
        bitmap.setPixels(pixels, 0, width, 0, 0, width, height);
        return bitmap;
```
... and it's 40 times faster !
Fore reference, I also tried another one, adapted from [here](http://androiducl.blogspot.fr/2012/04/bitmap-in-zxing.html): 
```java
        int BLACK = 0xFF000000;
        int WHITE = 0xFFFFFFFF;
        int width = matrix.getWidth();
        int height = matrix.getHeight();
        int[] pixels = new int[width * height];

        for (int y = 0; y < height; y++) {
            int offset = y * width;
            for (int x = 0; x < width; x++) {
                pixels[offset + x] = bitMatrix.get(x, y) ? BLACK : WHITE;
            }
        }

        Bitmap image = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888);
        image.setPixels(pixels, 0, width, 0, 0, width, height);
        return image;
```
It's 'only' 6 times faster.