问如何改变android设备的屏幕对比度？

EN

 Settings.System.putInt(this.getContentResolver(),
        Settings.System.SCREEN_BRIGHTNESS, 20);

WindowManager.LayoutParams lp = getWindow().getAttributes();
lp.screenBrightness =0.2f;// 100 / 100.0f;
getWindow().setAttributes(lp);

startActivity(new Intent(this,RefreshScreen.class));

boolean isFrontOn;
ImageButton btnFront;
SeekBar brightnessBar;
float brightness;
ContentResolver cResolver;
RelativeLayout overlayLayout;
String permissions[] = {"android.permission.WRITE_SETTING"};

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    btnFront = (ImageButton) findViewById(R.id.btnFront);
    brightnessBar = (SeekBar) findViewById(R.id.brightnessBar);

    cResolver = getContentResolver();
    brightnessBar.setMax(255);
    brightnessBar.setKeyProgressIncrement(1);

    btnFront.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            if (!isFrontOn)
                turnFrontOn();
            else
                turnFrontOff();
        }
    });

    try {
        brightness = Settings.System.getInt(cResolver, Settings.System.SCREEN_BRIGHTNESS);
    } catch (Settings.SettingNotFoundException e) {
        e.printStackTrace();
    }

    int screenBrightness = (int) brightness;
    brightnessBar.setProgress(screenBrightness);

    brightnessBar.setOnSeekBarChangeListener(new SeekBar.OnSeekBarChangeListener() {
        int progressVal = 0;

        @Override
        public void onProgressChanged(SeekBar seekBar, int progress, boolean fromUser) {

            if (progress <= 20)
                progressVal = 20;
            else
                progressVal = progress;
        }

        @Override
        public void onStartTrackingTouch(SeekBar seekBar) {

        }

        @Override
        public void onStopTrackingTouch(SeekBar seekBar) {
            if (Build.VERSION.SDK_INT < 23 || checkSelfPermission(permissions[0]) != PackageManager.PERMISSION_DENIED)
                Settings.System.putInt(cResolver, Settings.System.SCREEN_BRIGHTNESS, progressVal);
            else requestPermissions(permissions, 0);

        }
    });
}

@Override
public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
    super.onRequestPermissionsResult(requestCode, permissions, grantResults);
    if ( checkSelfPermission(permissions[0]) == PackageManager.PERMISSION_GRANTED) {
        //ToDo your code here after checking permission
        /*
        i.e)  Settings.System.putInt(cResolver, Settings.System.SCREEN_BRIGHTNESS, progressVal);
         */
    }
}