(#18440) Don't make class methods private

Previously, the launchd service provider `enabled?` instance method was
calling the `launchd_overrides` private class method, which is not
allowed. This issue wasn't noticed, because the spec test was overly
stubbing the provider.

This commit makes the class methods public to avoid these sorts of
problems in the future.
Merge pull request #1384 from joshcooper/ticket/master/18440-macosx-service

(#18440) Don't make class methods private