make `WriteConfig` and `GetConfig` private

[szaimen]

The WriteConfig and GetConfig functions are currently accessed in some files outside ConfigurationManager. In best case the functions should only be used by class internal functions.

See

• https://github.com/search?q=repo%3Anextcloud%2Fall-in-one+WriteConfig+-path%3A%2F%5Ephp%5C%2Fsrc%5C%2FData%5C%2F%2F&type=code
• https://github.com/search?q=repo%3Anextcloud%2Fall-in-one+GetConfig+-path%3A%2F%5Ephp%5C%2Fsrc%5C%2FData%5C%2F%2F&type=code