code: private synchronized void onAdFetched(NativeAd adNative) {
          Log.i(TAG, "onAdFetched");
          int index = -1;
          if (canUseThisAd(adNative)) {
              mPrefetchedAdList.add(adNative);
              index = mPrefetchedAdList.size()-1;
              mNoOfFetchedAds++;
          }
          lockFetch.set(false);
          mFetchFailCount = 0;
          ensurePrefetchAmount();
          onAdLoaded(index);
      }
docs: A handler for received native ads