how to pop up window/notification on startup of android

SystemUI is responsible to display the notification and alters and dialog windows with out depend on Activity. If you want to display an Alert after the boot up of android then add a peace of your code in
SystemUI apk. If any Intent is broadcasted then SystemUI broadcast receiver will receive that intent and call your function,
for  example you want to display an alert after boot completed then add code at where the Boot_COMPLETED intent should receive in SystemUI