要实现安卓自动防止屏幕变暗的功能，可以使用以下代码示例：

1. 创建一个BrightnessManager类，用于管理屏幕亮度的自动调节。

public class BrightnessManager {
    private Context mContext;
    private ContentResolver mContentResolver;
    private Handler mHandler;
    private Runnable mRunnable;

    public BrightnessManager(Context context) {
        mContext = context;
        mContentResolver = mContext.getContentResolver();
        mHandler = new Handler();
        mRunnable = new Runnable() {
            @Override
            public void run() {
                setAutoBrightness();
            }
        };
    }

    public void start() {
        mHandler.post(mRunnable);
    }

    public void stop() {
        mHandler.removeCallbacks(mRunnable);
    }

    private void setAutoBrightness() {
        try {
            // 检查是否开启了自动亮度调节
            if (Settings.System.getInt(mContentResolver, Settings.System.SCREEN_BRIGHTNESS_MODE)
                    == Settings.System.SCREEN_BRIGHTNESS_MODE_AUTOMATIC) {
                // 关闭自动亮度调节
                Settings.System.putInt(mContentResolver, Settings.System.SCREEN_BRIGHTNESS_MODE,
                        Settings.System.SCREEN_BRIGHTNESS_MODE_MANUAL);
            }

            // 获取当前亮度值
            int currentBrightness = Settings.System.getInt(mContentResolver,
                    Settings.System.SCREEN_BRIGHTNESS);

            // 设置亮度值为最大值
            Settings.System.putInt(mContentResolver, Settings.System.SCREEN_BRIGHTNESS,
                    255);

            // 应用亮度设置
            WindowManager.LayoutParams layoutParams = ((Activity) mContext).getWindow().getAttributes();
            layoutParams.screenBrightness = 1.0f;
            ((Activity) mContext).getWindow().setAttributes(layoutParams);

            // 恢复原来的亮度值
            Settings.System.putInt(mContentResolver, Settings.System.SCREEN_BRIGHTNESS,
                    currentBrightness);
        } catch (Settings.SettingNotFoundException e) {
            e.printStackTrace();
        }
    }
}

2. 在需要自动防止屏幕变暗的地方，调用BrightnessManager类的start()方法来启动亮度调节。

BrightnessManager brightnessManager = new BrightnessManager(this);
brightnessManager.start();

3. 在不需要自动防止屏幕变暗的地方，调用BrightnessManager类的stop()方法来停止亮度调节。

brightnessManager.stop();

这样，当启动了亮度调节后，屏幕亮度将会自动调整为最亮，并在一段时间后恢复原来的亮度值。