How to run an android app in background?

You can probably start a Service here if you want your Application to run in Background. This is what Service in Android are used for – running in background and doing longtime operations.

UDPATE

You can use START_STICKY to make your Service running continuously.

@Override
public int onStartCommand(Intent intent, int flags, int startId) {
    handleCommand(intent);
    // We want this service to continue running until it is explicitly
    // stopped, so return sticky.
    return START_STICKY;
}