DefaultCredentialsProvider is detecting AppEngine when running on GAE Flex #89

DefaultCredentialsProvider is detecting App Engine when running on App Engine Flex compat mode. Since GAE Flex is based on GCE VMs is would be better if it use ComputeEngineCredentials when running on Flex.

The App Engine check is here:
https://github.com/google/google-auth-library-java/blob/master/oauth2_http/java/com/google/auth/oauth2/DefaultCredentialsProvider.java#L211