code: public synchronized void destroyAllAds() {
          mFetchingAdsCnt = 0;
          adMapAtIndex.clear();
          mPrefetchedAdList.clear();
  
          Log.i(TAG, "destroyAllAds adList " + adMapAtIndex.size() + " prefetched " +
                  mPrefetchedAdList.size());
  
          super.destroyAllAds();
      }
docs: Destroys ads that have been fetched, that are still being fetched and removes all resource
  references that this instance still has. This should only be called when the Activity that
  is showing ads is closing, preferably from the {@link android.app.Activity#onDestroy()}.
  </p>
  The converse of this call is {@link #prefetchAds(Context)}.