public void deleteChannel(String channelId) {
        // Channels are supported since Android 8
        if (SDK_INT < O) return;

        Log.d(TAG, "Delete channel, id=" + channelId);

        // Cancel all notifications regarding this channel
        for (Notification notification : new Manager(context).getNotificationsFromSharedPreferences()) {
            if (notification.getOptions().getAndroidChannelId().equals(channelId)) {
                notification.cancel();
            }
        }

        NotificationManagerCompat.from(context).deleteNotificationChannel(channelId);
    }
Deletes a notification channel by an id. If you create a new channel with this same id,
the deleted channel will be un-deleted with all of the same settings it had before it was deleted.
@param channelId Like "my_channel_01"
src/android/Manager.java