addAdId method
Registers adId to watch for ad events (ready / fail / size).
Calling this with an already-registered adId updates the callback.
Implementation
void addAdId(String adId, AdCallback callback) {
_callbacks[adId] = callback;
if (!_adIds.contains(adId)) {
_adIds.add(adId);
} else {
Logger.w("$adId already added, updating callback");
}
}