Unresolved reference: default_web_client_id

### Step 1: Describe your environment

* Android device: n/a
* Android OS version: n/a
* Google Play Services version: `20.7.0`
* Firebase/Play Services SDK version: `32.7.0`

### Step 2: Describe the problem:

When I build the `auth` quickstart at 2066256a5c5681e339e31005647d95fcbabde030, I get the following error:

```
quickstart-android/auth/app/src/main/java/com/google/firebase/quickstart/auth/kotlin/GoogleSignInFragment.kt:122:51 Unresolved reference: default_web_client_id
```

AFAICT the Google Services Gradle plugin is supposed to generate this string in `values.xml` but for some reason isn't doing so.

#### Steps to reproduce:

Build the project at `2066256a5c5681e339e31005647d95fcbabde030`.

#### Observed Results:

See above.

#### Expected Results:

I expected the project to build.

#### Relevant Code:

n/a

Costruisci il quickstart dell'autenticazione al commit 2066256a5c5681e339e31005647d95fcbabde030 ed esamina GoogleSignInFragment.kt intorno alla riga 122 insieme al values.xml generato. Verifica perché default_web_client_id non è disponibile e conferma successivamente che il progetto venga compilato correttamente.

Scritto dal modello di indicizzazione a partire dal testo della issue.