Adjust to non-deprecated names in Keyring 1.6.+

Keyring 1.1 moved the concrete backend implementations into their
own modules. As we depend on 1.6.1+, we can make use of the new
name and remove the old one without deprecation-fallback.

Change-Id: I0682b13fc9f488b3f3d9fd057f712909fcd48bc4