Remove all notifications from notification bar

Starting with API Level 18 you can cancel Notifications posted by other apps different than your own using NotificationListenerService, the method changed a little in Lollipop, here is the way to remove notifications covering also Lillipop API.

First inside the onNotificationPosted method you store all the StatusBarNotification objects. Then you should maintain an updated reference to those objects, removing them if the notification is somehow dismissed in onNotificationRemoved method.

@TargetApi(Build.VERSION_CODES.JELLY_BEAN_MR2)
public class NotificationService extends NotificationListenerService {

    @Override
    public void onNotificationPosted(StatusBarNotification sbn) {
        // Store each StatusBarNotification object
    }

    @Override
    public void onNotificationRemoved(StatusBarNotification sbn) {
        // Delete removed StatusBarNotification objects
    }
}

Finally you can remove any notification using cancelNotification method.

 if (Build.VERSION.SDK_INT < Build.VERSION_CODES.LOLLIPOP) {
    cancelNotification(sbn.getPackageName(), sbn.getTag(), sbn.getId());
}
else {
    cancelNotification(sbn.getKey());
}

To remove all notifications:

Kotlin

val NM = getSystemService(AppCompatActivity.NOTIFICATION_SERVICE) as NotificationManager
NM.cancelAll()

Java

NotificationManager NM = getSystemService(AppCompatActivity.NOTIFICATION_SERVICE) 
NM.cancelAll()