code: @NotNull
      public List<String> getAllOwnedSkus(String itemType) {
          List<String> result = new ArrayList<String>();
          for (Purchase p : mPurchaseMap.values()) {
              if (p.getItemType().equals(itemType)) result.add(p.getSku());
          }
          return result;
      }
docs: Returns a list of all owned product IDs of a given type