FirebaseApp.Create not override default instance

### Please fill in the following fields:
Unity editor version: 2018.421f
Firebase Unity SDK version: 6.12.0
Source you installed the SDK : unitypackage
Firebase plugins in use : Auth FireStore
Additional SDKs you are using : Facebook AdMob VoxelBusters
Platform you are using the Unity editor on : Windows
Platform you are targeting : Android
Scripting Runtime : IL2CPP

### Please describe the issue here:
(Please list the full steps to reproduce the issue. Include device logs, Unity logs, and stack traces if available.)

```C#
FirebaseApp.LogLevel = Firebase.LogLevel.Verbose;
var status = await FirebaseApp.CheckAndFixDependenciesAsync();
var main = FirebaseApp.Create(new AppOptions() {
ProjectId = "{another}",
StorageBucket = "{anotherBucket}"
});
```

With the code above, the `main.Options.ProjectId` and `DefaultInstance` still being the ID from `config.json`. Is it a bug or is it intended?

### Please answer the following, if applicable:
Have you been able to reproduce this issue with just the Firebase Unity quickstarts (this GitHub project)?
- Don't know

What's the issue repro rate? (eg 100%, 1/5 etc)
- 100%