How to force an image to be loaded as ALPHA_8 bitmap on Android with BitmapFactory or ImageDecoder?

My goal is to upload a GL_ALPHA texture to the GPU and use it with OpenGL. For that, I need a bitmap in ALPHA_8 format.

Currently, I'm using BitmapFactory to load a 8-bit (with grayscale palette) PNG but getConfig() says it's in ARGB_8888 format.

I ended up using the PNGJ library, as follows:

import ar.com.hjg.pngj.IImageLine;
import ar.com.hjg.pngj.ImageLineHelper;
import ar.com.hjg.pngj.PngReader;

public static Bitmap loadAlpha8Bitmap(Context context, String fileName) {
    Bitmap result = null;

    try {
        PngReader reader = new PngReader(context.getAssets().open(fileName));

        if (reader.imgInfo.channels == 3 && reader.imgInfo.bitDepth == 8) {
            int size = reader.imgInfo.cols * reader.imgInfo.rows;
            ByteBuffer buffer = ByteBuffer.allocate(size);

            for (int row = 0; row < reader.imgInfo.rows; row++) {
                IImageLine line = reader.readRow();

                for (int col = 0; col < reader.imgInfo.cols; col++) {
                    int pixel = ImageLineHelper.getPixelRGB8(line, col);
                    byte gray = (byte)(pixel & 0x000000ff);
                    buffer.put(row * reader.imgInfo.cols + col, gray);
                }
            }

            reader.end();

            result = Bitmap.createBitmap(reader.imgInfo.cols, reader.imgInfo.rows, Bitmap.Config.ALPHA_8);
            result.copyPixelsFromBuffer(buffer);
        }
    } catch (IOException e) {}

    return result;
}