我最终使用了PNGJ library，如下所示：

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;
}