Make constructor for MediaLibraryState private

Problem/Motivation

The constructor for \Drupal\media_library\MediaLibraryState should not be public, since it is not a valid way to create the object, since the create method is a static factory.

It looks like this is just an omission from the initial implementation.

Steps to reproduce

Proposed resolution

Swap out public for private, in \Drupal\media_library\MediaLibraryState::__construct

Remaining tasks

Do it.

User interface changes

None

API changes

Close off an unused and invalid API.

Data model changes

None

Release notes snippet

Not required.