credentialManager: Shut up a JS warning

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)

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1512>