autorun: change logic for ignoring volumes

Previously, a volume was being ignored from autorun if one of these two
conditions were met:
- its mount root file had a native scheme and was mounted in a
  non-hidden location
- it had a volume that could have been automounted, and had a flag set
  by the shell to allow autorun

In order to effectively ignore volumes that we don't mount ourselves
from our notification system, we have to meet both conditions at the
same time instead.

https://bugzilla.gnome.org/show_bug.cgi?id=660595