How to kill or return focus from an android application?

Question

Android

I launch someone else's MyApp from my application:
Android.Content.Intent intent = PackageManager.GetLaunchIntentForPackage("com.app.app1");
StartActivity(intent);
Next, I would like to launch the second app2, but the system, apparently, takes the focus on app1 and everything stops there. How to kill app1 or return focus to MyApp?