Support Firebase SDK on devices without Google Play Services

I'm using unity 2018.3.0b2 and the firebase sdk 5.3.1 to build on my Oculus Go.

At startup I get this in my debug:
`Firebase App initializing app com.eyeflite.wingvr`

That's the last I see from my firebase debug output (no auth initiated or sign in attempt made). Then, I'm prompted with a system dialog that " {my app} won't run without play services which weren't supported by your device"

I hadn't seen this system prompt on 5.3.0 and the auth system was working fine. Any ideas about what's going on?

This is in my Awake() function:
```
Firebase.FirebaseApp.CheckAndFixDependenciesAsync().ContinueWith(task => {
var dependencyStatus = task.Result;
if (dependencyStatus == Firebase.DependencyStatus.Available) {
// Create and hold a reference to your FirebaseApp, i.e.
app = Firebase.FirebaseApp.DefaultInstance;
auth = Firebase.Auth.FirebaseAuth.DefaultInstance;
Debug.Log("auth inited!");
SignInAndGetToken();

} else {
UnityEngine.Debug.LogError(System.String.Format(
"Could not resolve all Firebase dependencies: {0}", dependencyStatus));
// Firebase Unity SDK is not safe to use here.
}
});
```