How to turn device screen off while incoming call in android?

Question

I am working on android app in which I want to turn device screen off during any incoming call.

I am using the following code to detect an incoming call.

public class MyCallReceiver extends BroadcastReceiver {

    @Override
    public void onReceive(Context context, Intent intent) {

        if (intent.getStringExtra(TelephonyManager.EXTRA_STATE).equals(
                TelephonyManager.EXTRA_STATE_RINGING)) {
            // This code will execute when the phone has an incoming call

            // get the phone number
            String incomingNumber = intent
                    .getStringExtra(TelephonyManager.EXTRA_INCOMING_NUMBER);
            Toast.makeText(context, "Call from:" + incomingNumber,
                    Toast.LENGTH_LONG).show();

            // here I need to setting device screen off

        } else if (intent.getStringExtra(TelephonyManager.EXTRA_STATE).equals(
                TelephonyManager.EXTRA_STATE_IDLE)
                || intent.getStringExtra(TelephonyManager.EXTRA_STATE).equals(
                        TelephonyManager.EXTRA_STATE_OFFHOOK)) {
            // This code will execute when the call is disconnected
            Toast.makeText(context, "Detected call hangup event",
                    Toast.LENGTH_LONG).show();

        }
    }
}

I searched and got this code which turns off the device screen. I implemented it inside my code, but without success and I cannot turn the device screen off programmatically during an incoming call yet.

Can anybody help me regarding this?

Edit:
The manifest file is like this:

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.demo"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="8"
        android:targetSdkVersion="15" />

    <uses-permission android:name="android.permission.READ_PHONE_STATE" />
    <uses-permission android:name="android.permission.WAKE_LOCK" />
    <uses-permission android:name="android.permission.DEVICE_POWER" />
    <uses-permission android:name="android.permission.MODIFY_PHONE_STATE" />


    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name="com.demo.MainActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

        <receiver android:name="com.demo.MyCallReceiver" >
            <intent-filter>
                <action android:name="android.intent.action.PHONE_STATE" />
            </intent-filter>
        </receiver>
    </application>

</manifest>