Fido app: Missing or invalid authentication

When starting the Fido app you can sign in using a Google account and afterwards logcat shows the following error
```
GAEService: Error extracting information from server's registration request
GAEService: com.google.api.client.googleapis.json.GoogleJsonResponseException: 503 Service Unavailable
GAEService: GET https://webauthndemo.appspot.com/_ah/api/fido2RequestHandler/v1/get/register
GAEService: {
GAEService: "code": 503,
GAEService: "errors": [
GAEService: {
GAEService: "domain": "global",
GAEService: "message": "com.google.appengine.api.datastore.DatastoreFailureException: Missing or invalid authentication.",
GAEService: "reason": "backendError"
GAEService: }
GAEService: ],
GAEService: "message": "com.google.appengine.api.datastore.DatastoreFailureException: Missing or invalid authentication."
GAEService: }
GAEService: at com.google.api.client.googleapis.services.json.AbstractGoogleJsonClientRequest.newExceptionOnError(AbstractGoogleJsonClientRequest.java:113)
GAEService: at com.google.api.client.googleapis.services.json.AbstractGoogleJsonClientRequest.newExceptionOnError(AbstractGoogleJsonClientRequest.java:40)
GAEService: at com.google.api.client.googleapis.services.AbstractGoogleClientRequest$1.interceptResponse(AbstractGoogleClientRequest.java:321)
GAEService: at com.google.api.client.http.HttpRequest.execute(HttpRequest.java:1111)
GAEService: at com.google.api.client.googleapis.services.AbstractGoogleClientRequest.executeUnparsed(AbstractGoogleClientRequest.java:419)
GAEService: at com.google.api.client.googleapis.services.AbstractGoogleClientRequest.executeUnparsed(AbstractGoogleClientRequest.java:352)
GAEService: at com.google.api.client.googleapis.services.AbstractGoogleClientRequest.execute(AbstractGoogleClientRequest.java:469)
GAEService: at com.fido.example.fido2apiexample.GAEService.getRegistrationRequest(GAEService.java:105)
GAEService: at com.fido.example.fido2apiexample.Fido2DemoActivity$11.call(Fido2DemoActivity.java:472)
GAEService: at com.fido.example.fido2apiexample.Fido2DemoActivity$11.call(Fido2DemoActivity.java:468)
GAEService: at com.google.android.gms.tasks.zzy.run(com.google.android.gms:play-services-tasks@@17.2.0:2)
GAEService: at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167)
GAEService: at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641)
GAEService: at java.lang.Thread.run(Thread.java:923)
Fido2DemoActivity: Register request is null
```

失敗経路は GAEService.java:105 と Fido2DemoActivity.java:468-472 に示されています。まずサインインフローを再現し、registration-request の呼び出しとその null 結果を調査してください。Google サインイン後に Fido アプリが有効な registration request を取得し、記録された 503 や「Register request is null」の出力が発生しなければ完了です。

索引モデルが issue の本文から書いたものです。