ActivityManager.getRunningTasks is deprecated android

This should help you get started

ActivityManager activityManager = (ActivityManager) context.getSystemService(Context.ACTIVITY_SERVICE);
List<ActivityManager.AppTask> tasks = activityManager.getAppTasks();

for (ActivityManager.AppTask task : tasks) {
    Log.d(TAG, "stackId: " + task.getTaskInfo().stackId);
}

This should work for the pre Lollipop devices as well as for Lollipop devices

public static boolean isBackgroundRunning(Context context) {
        ActivityManager am = (ActivityManager) context.getSystemService(Context.ACTIVITY_SERVICE);
        List<ActivityManager.RunningAppProcessInfo> runningProcesses = am.getRunningAppProcesses();
        for (ActivityManager.RunningAppProcessInfo processInfo : runningProcesses) {
            if (processInfo.importance == ActivityManager.RunningAppProcessInfo.IMPORTANCE_FOREGROUND) {
                for (String activeProcess : processInfo.pkgList) {
                    if (activeProcess.equals(context.getPackageName())) {
                        //If your app is the process in foreground, then it's not in running in background
                        return false;
                    }
                }
            }
        }


        return true;
    }

Edit: It should return true if the app is in background, not the opposite