Ordinarily one should let the OS decide when an Android app gets killed. But you can call ExitApplication.exit() to explicitly kill the running application:

public class ExitApplication {
public static void exit() {
int processID = android.os.Process.myPid();
android.os.Process.killProcess(processID);
}
}

Use Vault 3 for Android to keep track of your important notes when you're on the go. Thanks!