Android - Rotate line so it stays parallel to the ground

Question

I am experimenting with the accelerometer and the gyroscope. I have defined a simple line View in my layout.xml and i want to calculate the current angle of the phone so i can use View.SetRotation() to have the line be parallel to the floor. I have found many tutorials but i can't get it to work.

Here is my code:

layout.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin"
    tools:context="com.example.main.camera3doit.TestActivity">

    <TextView
        android:text="angle"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/angleText"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true"
        android:textColor="#ffff0000"
        android:textSize="30dp" />

    <View
        android:id="@+id/lineView"
        android:layout_width="match_parent"
        android:layout_height="2dp"
        android:layout_centerVertical="true"
        android:layout_centerHorizontal="true"
        android:background="#ff01eb00" />

</RelativeLayout>

TestActivity.java

public class TestActivity extends Activity {
    TextView angleText;
    View line;

    private SensorManager sensorManager;
    private Sensor accelerometer;
    private Sensor magnetometer;

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


        sensorManager = (SensorManager)getSystemService(SENSOR_SERVICE);
        accelerometer = sensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER);
        magnetometer = sensorManager.getDefaultSensor(Sensor.TYPE_MAGNETIC_FIELD);

        line = findViewById(R.id.lineView);
        angleText = (TextView) findViewById(R.id.angleText);
    }

    protected void onResume() {
        super.onResume();
        sensorManager.registerListener(sensorEventListenerer, accelerometer, SensorManager.SENSOR_DELAY_NORMAL);
        sensorManager.registerListener(sensorEventListenerer, magnetometer, SensorManager.SENSOR_DELAY_NORMAL);
    }

    protected void onPause() {
        super.onPause();
        sensorManager.unregisterListener(sensorEventListenerer);
    }


    private SensorEventListener sensorEventListenerer = new SensorEventListener() {
        @Override
        public void onSensorChanged(SensorEvent event) {

            /*
            *
            *
            *
            * Here i want to do my calculations 
            * so i can find what rotation the line should have
            *
            *
            *
            *
             */

            angleText.setText(phoneRotationDegrees);
            line.setRotation(DegreesSoItIsParallelToTheGround);
        }

        @Override
        public void onAccuracyChanged(Sensor sensor, int accuracy) {
        }
    };
}