Wake Android Device up

To wake up the screen:

PowerManager pm = (PowerManager) getApplicationContext().getSystemService(Context.POWER_SERVICE);
WakeLock wakeLock = pm.newWakeLock((PowerManager.SCREEN_BRIGHT_WAKE_LOCK | PowerManager.FULL_WAKE_LOCK | PowerManager.ACQUIRE_CAUSES_WAKEUP), "TAG");
wakeLock.acquire();

To release the screen lock:

KeyguardManager keyguardManager = (KeyguardManager) getApplicationContext().getSystemService(Context.KEYGUARD_SERVICE); 
KeyguardLock keyguardLock = keyguardManager.newKeyguardLock("TAG");
keyguardLock.disableKeyguard();

And the manifest needs to contain:

<uses-permission android:name="android.permission.WAKE_LOCK" />
<uses-permission android:name="android.permission.DISABLE_KEYGUARD" />

For more details about PowerManager, refer to the API documentation: http://developer.android.com/reference/android/os/PowerManager.html

EDIT: this answer is reported as deprecated.

I found a way and it is not that complex... works on any API version.

You need to use PowerManager.userActivity(l, false) method and register your activity as broadcast received for SCREEN_OFF intent:

In your actiivity OnCreate put something like:

mReceiver = new BroadcastReceiver() {
    @Override
    public void onReceive(Context context, Intent intent) {
        Log.v(TAG, "Screen OFF onReceive()");
        screenOFFHandler.sendEmptyMessageDelayed(0, 2000L);
    }
};

It will kick off the handler after 2 seconds of Screen Off event.

Register receiver in your onResume() method:

IntentFilter filter = new IntentFilter(Intent.ACTION_SCREEN_OFF);
registerReceiver(mReceiver, filter);
Log.i(TAG, "broadcast receiver registered!");

Create a handler like the one below:

private Handler screenOFFHandler = new Handler() {

    @Override
    public void handleMessage(Message msg) {

        super.handleMessage(msg);
        // do something
        // wake up phone
        Log.i(TAG, "ake up the phone and disable keyguard");
        PowerManager powerManager = (PowerManager) YourActivityName.this
                .getSystemService(Context.POWER_SERVICE);
        long l = SystemClock.uptimeMillis();
        powerManager.userActivity(l, false);//false will bring the screen back as bright as it was, true - will dim it
    }
};

Request permission in your manifest file:

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

Do not forget to unregister broadcast receiver when you are done. You may do that in onDestroy() for example (which is not guaranteed)

unregisterReceiver(mReceiver);
Log.i(TAG, "broadcast UNregistred!");

On newer devices you should use something like this, since the mentioned Flags are deprecated.

class AlarmActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {

        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_alarm)

        // Keep screen always on, unless the user interacts (wakes the mess up...)
        window.addFlags(WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON)

        setTurnScreenOn(true)
        setShowWhenLocked(true)

        (getSystemService(KeyguardManager::class.java) as KeyguardManager).requestDismissKeyguard(this,
            object: KeyguardManager.KeyguardDismissCallback(){
                override fun onDismissCancelled() {
                    Log.d("Keyguard", "Cancelled")
                }

                override fun onDismissError() {
                    Log.d("Keyguard", "Error")
                }

                override fun onDismissSucceeded() {
                    Log.d("Keyguard", "Success")
                }
            }
        )
    }
}

KeyguardManager.requestDismissKeyguard only wakes up the device, if the setter setTurnScreenOn(true) was called before.

I tested this on my Android Pie device.

Try with the below code after setContentView(R.layout.YOUR_LAYOUT); in activity onCreate() method

    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O_MR1) {
        Log.d(TAG, "onCreate: set window flags for API level > 27");
        getWindow().addFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN
                        | WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON);
        KeyguardManager keyguardManager = (KeyguardManager) getApplicationContext().getSystemService(Context.KEYGUARD_SERVICE);
        keyguardManager.requestDismissKeyguard(this, null);
        setShowWhenLocked(true);
        setTurnScreenOn(true);
    } else {
        Log.d(TAG, "onCreate: onCreate:set window flags for API level < 27");
        getWindow().addFlags(
                WindowManager.LayoutParams.FLAG_FULLSCREEN
                        | WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON
                        | WindowManager.LayoutParams.FLAG_DISMISS_KEYGUARD
                        | WindowManager.LayoutParams.FLAG_SHOW_WHEN_LOCKED
                        | WindowManager.LayoutParams.FLAG_TURN_SCREEN_ON);
    }