Q:

How to know when my app has been killed?

I need to know when the user kills my app (Force stop). I've been reading the android lifecycle, which has the onStop() and onDestroy() functions, these are related to each activity the user ends on my app, but not when the user forces stop or kills my app.
Is there any way to know when the user kills the app?

A:

I have found one way to do this.....

Make one service like this  
public class OnClearFromRecentService extends Service {

    @Override
    public IBinder onBind(Intent intent) {
        return null;
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        Log.d("ClearFromRecentService", "Service Started");
        return START_NOT_STICKY;
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        Log.d("ClearFromRecentService", "Service Destroyed");
    }

    @Override
    public void onTaskRemoved(Intent rootIntent) {
        Log.e("ClearFromRecentService", "END");
        //Code here
        stopSelf();
    }
}

Register this service in Manifest.xml  like this 
<service android:name="com.example.OnClearFromRecentService" android:stopWithTask="false" />

Then start this service on your splash activity 
startService(new Intent(getBaseContext(), OnClearFromRecentService.class));

And now whenever you will clear your app from android recent Then this method  onTaskRemoved() will execute.
NOTE: In Android O+ this solution only works when the app is full-time in foreground. After more than 1 minute with the app in background, the OnClearFromRecentService (and all other Services running) will be automatically force-killed by the system so the onTaskRemoved() will not be executed. 

A:

there's no way to determine when a process is killed.
From How to detect if android app is force stopped or uninstalled?

When a user or the system force stops your application, the entire
  process is simply killed. There is no callback made to inform you that
  this has happened.
When the user uninstalls the app, at first the process is killed, then
  your apk file and data directory are deleted, along with the records
  in Package Manager that tell other apps which intent filters you've
  registered for.

A:

Create a Application class
onCreate()
Called when the application is starting, before any activity, service, or receiver objects (excluding content providers) have been created.
onLowMemory()
This is called when the overall system is running low on memory, and actively running processes should trim their memory usage.
onTerminate()
This method is for use in emulated process environments.

Even if you are application Killed or force stop, again Android will start your Application class  

