code: public GVRAndroidResource addResource(GVRAndroidResource resource)
      {
          String fileName = resource.getResourcePath();
          GVRAndroidResource resourceValue = resourceMap.get(fileName);
          if (resourceValue != null)
          {
              return resourceValue;
          }
          // Only put the resourceKey into the map for the first time, later put
          // will simply return the first resource
          resourceValue = resourceMap.putIfAbsent(fileName, resource);
          return resourceValue == null ? resource : resourceValue;
      }
docs: Adds an android resource to the volume's resource map.
  This function is useful if you have created the GVRAndroidResource from a resource ID
  but would like to read it via it's filename. If you add the resource first,
  you can call openResource() on it's filename.
  
  @param resource
  Android resource to open.
  
  @throws IOException