Workaround for broken interstitial and rewarded ad callbacks in Xamarin.GooglePlayServices.Ads.

XamarinAdMobHack.cs

using System;
using Android.Runtime;

namespace Android.Gms.Ads.Hack
{
    public abstract class InterstitialAdLoadCallback : global::Android.Gms.Ads.Interstitial.InterstitialAdLoadCallback
    {
        private static Delegate cb_onAdLoaded;

        private static Delegate GetOnAdLoadedHandler()
        {
            if (cb_onAdLoaded is null)
            {
                cb_onAdLoaded = JNINativeWrapper.CreateDelegate((Action<IntPtr, IntPtr, IntPtr>)n_onAdLoaded);
            }
            return cb_onAdLoaded;
        }

        private static void n_onAdLoaded(IntPtr jnienv, IntPtr native__this, IntPtr native_p0)
        {
            InterstitialAdLoadCallback thisobject = GetObject<InterstitialAdLoadCallback>(jnienv, native__this, JniHandleOwnership.DoNotTransfer);
            global::Android.Gms.Ads.Interstitial.InterstitialAd resultobject = GetObject<global::Android.Gms.Ads.Interstitial.InterstitialAd>(native_p0, JniHandleOwnership.DoNotTransfer);
            thisobject.OnAdLoaded(resultobject);
        }

        [Register("onAdLoaded", "(Lcom/google/android/gms/ads/interstitial/InterstitialAd;)V", "GetOnAdLoadedHandler")]
        public virtual void OnAdLoaded(global::Android.Gms.Ads.Interstitial.InterstitialAd interstitialAd)
        {
        }
    }

    public abstract class RewardedAdLoadCallback : global::Android.Gms.Ads.Rewarded.RewardedAdLoadCallback
    {
        private static Delegate cb_onAdLoaded;

        private static Delegate GetOnAdLoadedHandler()
        {
            if (cb_onAdLoaded is null)
            {
                cb_onAdLoaded = JNINativeWrapper.CreateDelegate((Action<IntPtr, IntPtr, IntPtr>)n_onAdLoaded);
            }

            return cb_onAdLoaded;
        }

        private static void n_onAdLoaded(IntPtr jnienv, IntPtr native__this, IntPtr native_p0)
        {
            RewardedAdLoadCallback thisobject = GetObject<RewardedAdLoadCallback>(jnienv, native__this, JniHandleOwnership.DoNotTransfer);
            global::Android.Gms.Ads.Rewarded.RewardedAd resultobject = GetObject<global::Android.Gms.Ads.Rewarded.RewardedAd>(native_p0, JniHandleOwnership.DoNotTransfer);
            thisobject.OnAdLoaded(resultobject);
        }

        [Register("onAdLoaded", "(Lcom/google/android/gms/ads/rewarded/RewardedAd;)V", "GetOnAdLoadedHandler")]
        public virtual void OnAdLoaded(global::Android.Gms.Ads.Rewarded.RewardedAd rewardedAd)
        {
        }
    }
}

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Updated to not require unsafe code compiler flag. See xamarin/GooglePlayServicesComponents#425 for details.