[google_sign_in_android] Keep `default_web_client_id` resource from being stripped by resource shrinker (#12075)

Adds a keep rule for `@string/default_web_client_id`, which gets merged into the main app's keep rules during the build.

Fixes https://github.com/flutter/flutter/issues/188363

### Why this is needed

As documented in [Customize which resources to keep](https://developer.android.com/topic/performance/app-optimization/customize-which-resources-to-keep):
> You should rarely need to keep resources. The use of getIdentifier() is an exception; however, retrieving resources by identifier is more efficient than retrieving them by name with getIdentifier().

The plugin retrieves `default_web_client_id` dynamically via `Resources.getIdentifier()` in [`GoogleSignInPlugin.java`](https://github.com/flutter/packages/blob/main/packages/google_sign_in/google_sign_in_android/android/src/main/java/io/flutter/plugins/googlesignin/GoogleSignInPlugin.java#L201-L204):

```java
int webClientIdIdentifier =
    context
        .getResources()
        .getIdentifier("default_web_client_id", "string", context.getPackageName());
```

Without this keep rule, Android's resource shrinker discards `@string/default_web_client_id` in release builds because there are no static bytecode references to the resource ID.