以最节省内存的方式读取本地图片文件，生成Bitmap对象

    public static Bitmap readBitMap(Context context, int resId) {
        BitmapFactory.Options opt = new BitmapFactory.Options();
        opt.inPreferredConfig = Bitmap.Config.RGB_565;
        opt.inPurgeable = true;
        opt.inInputShareable = true;
        //获取资源图片
        InputStream is = context.getResources().openRawResource(resId);
        return BitmapFactory.decodeStream(is,null,opt);
    }