[google-auth-library-java] AppEngineCredential depends on legacy bundled service

Hello, we are upgrading GAE from Java 8 to 11 and also trying to get ride of legacy bundled service .

Here is how we generate default AccessToken today.
```
GoogleCredentials credentials = AppEngineCredentials.newBuilder().setScopes(scopes).setAppIdentityService(AppIdentityServiceFactory.getAppIdentityService()).build();
```
The reason we do it this way is because if using `GoogleCredentials.getDefaulCredential`, because the call to .createScoped will silently proceed by doing nothing?

Question:
1: How to use AppEngineCredentials without turning on legacy bundle? Because I get feature not enabled error when calling without turning it on
2: Do we still need to include `appengine-sdk1.0` in our dependency? As those imports are there:
```
import com.google.appengine.api.appidentity.AppIdentityService;
import com.google.appengine.api.appidentity.AppIdentityService.GetAccessTokenResult;
import com.google.appengine.api.appidentity.AppIdentityServiceFactory;
```

Thanks