How to point GOOGLE_APPLICATION_CREDENTIALS to my JSON file?

I would like to use Google Text-to-Speech in my Android app.

According to documentation, I have been through all these steps:

1. In the Cloud Console, go to the Create service account key page.
2. From the Service account list, select New service account.
3. In the Service account name field, enter a name.
4. From the Role list, select Project > Owner.
5. Click Create. A JSON file that contains your key downloads to your computer.

I am using Google's code to pass the credential, which is the JSON file that I have downloaded:

public void main(String... args) throws Exception {
    // You can specify a credential file by providing a path to GoogleCredentials.
    // Otherwise credentials are read from the GOOGLE_APPLICATION_CREDENTIALS environment variable.
    InputStream stream = getResources().openRawResource(R.raw.credential); // R.raw.credential is credential.json
    GoogleCredentials credentials = GoogleCredentials.fromStream(new FileInputStream(String.valueOf(stream)))
            .createScoped(Lists.newArrayList(Collections.singleton("https://www.googleapis.com/auth/cloud-platform")));
    Storage storage = StorageOptions.newBuilder().setCredentials(credentials).build().getService();

    System.out.println("Buckets:");
    Page<Bucket> buckets = storage.list();
    for (Bucket bucket : buckets.iterateAll()) {
        System.out.println(bucket.toString());
    }
}

Anyway, I have cleaned Gradle, restarted Android Studio but I am still getting this log:

java.io.IOException: The Application Default Credentials are not available. They are available if running in Google Compute Engine. Otherwise, the environment variable GOOGLE_APPLICATION_CREDENTIALS must be defined pointing to a file defining the credentials. See https://developers.google.com/accounts/docs/application-default-credentials for more information.

What am I doing wrong?

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