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Show notification icon on status bar with React Native when app is not running on Android

I have a situation on my React Native app, where user can start a timer and isRunning and startTime states are stored in the app so that it can display current running time when the app is in foreground even though the user quits the application at some point and opens it again.
Is there a way to show notification icon on status bar when the timer is running, but user has quit the application to indicate that the timer is currently "running" (actually it is not doing any operations on background) on background?
I have encountered some apps that display a silent notification that is not directly closable after I have quit the app, but I haven't seen a situation where status bar notification is present (together with notification) until some condition is met after the app is quit by the user.
Is there a way to achieve this? I am using react-native-push-notification and Firebase to push notifications in my app overall.

React-Native manages only Active and Background/Inactive/Foreground state. When user kill the app. JS engine shuts down.
All you can use is Local Notification and Scheduled Notification. I also have one app in which user set a reminder time. And notification invoke at that specified time.
Using Firebase I guess you need to call api after some specific interval for push notification from server side.

I am not an expert in background services or android development but here is my thought... I believe you could dig in and write some java android code for a background unstoppable service (persistent on app close or background states). You can do this by creating a bridge and using native modules to manage your background services. I came across a great resource on medium that details the process of creating a background service, a broadcast receiver and maintaining Headless instance even when the app is closed or the device restarts.Hope that helps you achieve your goal.
Edit
This ready made package will help you with better. Check it out. You actually don't have to write native android java code at all.

How does Setting Up Notification when user leaves the app work?

I am new to coding. I am using Android Studio with Java. I spent the last 3 hours looking for how to set up a notification display at the top when the user leaves the app so they can always click on the notification when they want to return to the app but I couldn't find anything with that action I need.
The app tracks the users GPS and I want them to be able to always be aware that when the app is running it will continue to track their GPS. So my idea is to have a notification window pop up whenever they leave the app so they can be aware the app is running in the background.
Any help will be much appreciated! I am building this app on my own as a passion project and I am taking the Udacity Google Android course. I finished the course early and working on this app for fun. We didn't learn about Notifications or GPS tracking but I am doing my best to learn new things. Thank you for your patience and help!
My app code is here for easy viewing:
https://github.com/PoetryHorizon/eloteroMan2

Use a foreground Service. For any background logic, you should be using a Service anyway, and in Android Oreo and above, you're required to have it be a foreground Service, meaning it posts a Notification.
You can then add a PendingIntent to that notification set to reopen your app. See Android's Notification documentation on how to set a PendingIntent. You'll want to use PendingIntent.getActivity(), where the Intent you pass is new Intent(someContext, YourActivity.class).

Clarification for app to run in background using Service

I am trying to develop an app that should run in the background. I want my app to pop-up a message everyday at a particular time of day. As i read i will have to use 'Service' for this. http://developer.android.com/reference/android/app/Service.html#ProcessLifecycle This link shows an example. But where should my code for pop-up be? I dint understand well from the above link. Another question i have is will the app continue to run even after a reboot?

you can show a notification from the app to show pop up msgs, that is the best practice. though showing dialog is also possible. code will be in service only. see this link
And to keep your application running after reboot you will have to register for BOOT_COMPLETED broadcast and whenever you receive that broadcast, you will have to launch your service again. see this link

Android:Retain state of application when launch from background to foreground

I want to show notification of my application in the form of icon,whether it is run in background or foreground.If we launch application from notification then I want my applications state prior to application in background.

Android apps retain their state by default. There are a few exceptions to this, but normally its not a problem. To enable your application to do work in the background you should look into the Service class.
http://developer.android.com/guide/components/services.html

You can use the android notification from here
And you can use the service if you are developing and media applicaion.

Can I prevent the user from stopping my Android application?

The requirement is for an enterprise application. The application will be started on device boot. It will be running in the background and the user should not be able to disable or Stop the application. In Android a user can go to Settings->Application->Manage Application and stop my application. Is there any way to prevent this from happening?

No there is not. You can prevent Android from stopping the application by utilizing a Service and marking it as a foreground service, though this will require your application to display an icon in the status bar.

You can not make your application live forever, but it depends on what you really want to do. It's possible to receive a lot of events of the mobile and execute code even if your Activity/Service is not running. You can use BroadcastReceivers to look for interesting events and then start a service. I do it for an Enterprise Application that sends an event to a main server when the user has received/made a call.