[google-auth-library-java] ComputeEngineCredentials incorrectly throws RuntimeException

Unless we really think getDefaultServiceAccount can't fail here, we should not convert the IOException to a RuntimeException.

```
@Override
public String getAccount() {
if (serviceAccountEmail == null) {
try {
serviceAccountEmail = getDefaultServiceAccount();
} catch (IOException ex) {
throw new RuntimeException("Failed to to get service account", ex);
}
}
return serviceAccountEmail;
}
```