RingAccount: prevent insertion of empty file content in archive

Rewrite of RingAccount::updateArchive() to prevents
insertion of empty file contents in case of non-existing files.
This may cause later empty-object creation with misleading usage.
Also exposes filtered and base64 decoded keys to easier code
readability and maintainance.

Change-Id: Id2d504ad73caaaad519be0344c3fb424ada35c20
Reviewed-by: Andreas Traczyk <andreas.traczyk@savoirfairelinux.com>