[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This method calls doesn't seem to work anymore having been replaced by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 that GetSessionByPID no longer
works correctly, but we may as well port to the simpler path that avoids
so many layers of indirection.

Use of "/auto" does not exist on old distros so the legacy path is kept.

The paths used in this class were always wrong, which went unnoticed as
this is the first usage of them.

BUG: 423526