message: Do not create new "dex" when adding a generated class.
  Summary:
  As title.
  Initially, we assumed, InterDex would come along and clean-up this unecessary
  dex. We shouldn't assume this (although most of the app should have InterDex
  enabled).
diff: @@ -346,8 +346,8 @@ void add_class(DexClass* new_cls, Scope& scope, DexStoresVector& stores) {
  // TODO(emmasevastian): Handle this case in a better way.
  if (!stores.empty()) {
  DexClassesVector& root_store = stores.front().get_dexen();
  - DexClasses dc = {new_cls};
  - root_store.emplace_back(std::move(dc));
  + // Add the class to the last dex.
  + root_store.back().emplace_back(new_cls);
  }
  }
  