[java-logging] Publish LocalLoggingHelper, LocalLoggingImpl for unit testing of google-cloud-logging usage

We would like to do JUnit tests of our usage of the Stackdriver Logging API. There are some useful classes -- `LocalLoggingHelper`, `LocalLoggingImpl` -- that we could use, but they are not published in the google-cloud-logging package. This would be a much better solution for us than using the [test project](https://github.com/googleapis/google-cloud-java/blob/master/TESTING.md#testing-code-that-uses-logging) solution advocated for here.