For anyone else that runs into this issue, here is how I fixed it.
I was running Xamarin.Forms 2.4.0.282 and Xamarin.Forms.AppLinks 2.4.0.282 using Visual Studio 2015 Enterprise and Xamarin.Android 7.1.0.43 and everything was working just fine. My target is Android 7.1.
I upgraded to Visual Studio 2017 Enterprise and Xamarin.Android 8.2.0.16 (HEAD/a78295902). My target is Android 7.1. That's when I started seeing issues with Xamarin.Forms.AppLinks 2.4.0.282, Xamarin.GooglePlayServices.AppIndexing 29.0.0.1, Xamarin.GooglePlayServices.Base 29.0.0.1 and Xamarin.GooglePlayServices.Basement 29.0.0.1.
Occurring in the OnCreate method
Java.Lang.IllegalStateException: Default FirebaseApp is not initialized in this process com.domainname.appname. Make sure to call FirebaseApp.initializeApp(Context) first.
I upgraded to Xamarin.Forms.AppLinks 2.5.0.280555 and applied Pablo Biagioli's comment on 2017-07-07 15:52:15 UTC from this Bugzilla entry.
https://bugzilla.xamarin.com/show_bug.cgi?id=54383
It fixed the FirebaseApp.initializeApp(Context) issue, but I noticed that none of the new registered links were appearing in the Google Search. One of my old links was appearing in Google Search, but when I clicked on it, it pulled up the web page instead of Deep Linking into my app.
I then upgrade to Xamarin.Forms 2.5.1.444934, did a clean - rebuild and build. I now getting the error again.
Occurs when Application.Current.AppLinks.RegisterLink(_appLink); is used in the Page
Java.Lang.IllegalStateException: Default FirebaseApp is not initialized in this process com.domainname.appname. Make sure to call FirebaseApp.initializeApp(Context) first.
I set the Target to 8.0 and the error still occurred. I set the Target to 8.1 and the error has gone away.
Deep Linking is still not working like it was, so I'm not sure what is wrong. At least the error has gone away.