Moving more PDF encryption implementation into common encryption
files. This will make it easier to add new encryption later on as
the code will be in one place.
Change-Id: Id40c2f876a2e92bb8db27024a0e251befc5059e5
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/176030
Tested-by: Jenkins
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>
