System.exit()
does not kill your app if you have more than one activity on the stack
Use android.os.Process.killProcess(android.os.Process.myPid());
this way.
for sample
public void onBackPressed() {
android.os.Process.killProcess(android.os.Process.myPid());
super.onBackPressed();
}
For more detail see Is quitting an application frowned upon? , How to force stop my android application programmatically?
I hope this will help you.