loadAd method
Implementation
Future<void> loadAd() async {
print("fsfp_tag: BannerAd.dart. loadAd.");
_channel!.setMethodCallHandler(adsCallbackHandler);
Map params = FreestarUtils.paramsFrom(placement, targetingParams);
params["adSize"] = adSize;
unawaited (_channel!.invokeMethod('loadBannerAd', params));
}