credentialManager: Shut up a JS warning

Plain classes are private to their file, so accessing them from another module results in the following warning:

That property was defined with 'let' or 'const' inside the module.
This was previously supported, but is not correct according to the
ES6 standard.

Fix by assigning the class to a public variable instead.

(Eventually switching to ES6 modules with proper imports/exports will fix this as well)