startActivity not working when calling from BroadcastReceiver

An activity can be started from Broadcast Receiver even if it is not a launcher. That shouldn't be the problem.

Try passing a unique requestid,

notificationIntent = new Intent();
notificationIntent.setAction(AppConstants.ACTION_ACTIVITY);
notificationIntent.putExtra("key", value);

mBuilder.setContentIntent(PendingIntent.getBroadcast(context, value, notificationIntent, PendingIntent.FLAG_UPDATE_CURRENT));

See this question.

For someone, who will look for a similar question. Beginning from Android Q it's restricted to launch any activity, when app in background. To launch your activity, you need to displaying a notification with PRIORITY_HIGH and set the full-screen intent. So, totally you should have something like this:

//Create your intent as usual
Intent intent = new Intent(context, Activity.class);
intent.putExtra("key", value);
intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);

// Build a pending intent 
PendingIntent pIntent = PendingIntent.getActivity(context, 0,
                intent, PendingIntent.FLAG_UPDATE_CURRENT);

//Notification builder
NotificationCompat.Builder notifyBuilder = new NotificationCompat.Builder(context, CHANNEL_ID)
                .setSmallIcon(R.drawable.ic_notification)
                .setContentTitle(title)
                .setPriority(PRIORITY_HIGH)
                .setFullScreenIntent(pIntent, true /*isHighPriority*/);

NotificationManager nManager = (NotificationManager) context.getSystemService(Context.NOTIFICATION_SERVICE);
nManager.notify(10, notifyBuilder.build())

This will display notification and will launch your activity immediately. And in your activity onStart() you can dismiss a notification like below:

NotificationManager nManager = (NotificationManager) context.getSystemService(Context.NOTIFICATION_SERVICE);
nManager.cancel(10);

NOTE: Don't forget to init channel. Beginning from Android O it's a mandatory thing

in order to start activity from BroadcastReciever you should add flag of FLAG_ACTIVITY_NEW_TASK

 Intent imap =new Intent(context,MainActivity.class);
                  imap.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
                  context.startActivity(imap);

Try using flags as follows for the intent.

intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_MULTIPLE_TASK);

And for Pending intent use PendingIntent.FLAG_UPDATE_CURRENT