[libkworkspace] Port from deprecated GetSessionByPID
This method calls doesn't seem to work anymore having been replaced by the more intuitve virtual paths on the logind over a year ago.
Arguably that's still a bug upstream if GetSessionByPID no longer works correctly, but we may as well port to the simpler path that avoids so many layers of indirection and fix it for users.
The paths used in this class were always wrong, which went unnoticed as this is the first usage of them.