Listening to dbus signals to mount mtp device on debian linux

I am trying to write a python script that will detect when an mtp device (in this case an Android phone) has been mounted to a raspberry pi 3 (running raspbian jessie). I want to automatically transfer files to the phone when it is plugged in.

I started out with the script given as the second answer to this question

I changed a few things to get it to run on my raspberry pi. The current script is below. I can see UnitNew and UnitRemoved signals when I plug/unplug my phone. When it gets to the path_to_device function, I get this error:

DBusException: org.freedesktop.DBus.Error.ServiceUnknown: The name org.freedesktop.systemd1.Manager was not provided by any .service files

I read in a similar question that this is a permissions issue, so I tried running the script as sudo ./usb_SYSTEM.py but I still get the same error.

I have also tried using a SessionBus instead of a SystemBus. The signals are different, the bus name is then org.gtk.vfs.MountTracker. I still get the same error, this time it says org.gtk.vfs.MountTracker is not an available service. When I try to run the SessionBus version with sudo, I don't see the signals at all.

Am I using the right input to the get_object() function? And the right bus type? There is no destination bus since the signal is broadcast, I'm not sure if that is part of the problem. I have a very limited understanding of linux and signals/buses.

Script:

#!/usr/bin/env python
# -*- encoding: utf-8 -*-

import dbus
import dbus.service
if getattr(dbus, 'version', (0,0,0)) >= (0,41,0):
    import dbus.glib

from dbus.mainloop.glib import DBusGMainLoop
from gi.repository import GLib
import sys
import os


class DeviceManager:
    def __init__(self):
        self.bus = dbus.SystemBus()
        self.bus.add_signal_receiver(self.device_added,
                        'UnitNew',
                        'org.freedesktop.systemd1.Manager',
                        None,
                        '/org/freedesktop/systemd1',
                        path_keyword='path')

        self.bus.add_signal_receiver(self.device_removed,
                        'UnitRemoved',
                        'org.freedesktop.systemd1.Manager',
                        None,
                        '/org/freedesktop/systemd1',
                        path_keyword='path')

        self.bus.add_signal_receiver(self.print_event, None, None, None, None,
                                     sender_keyword='sender', message_keyword='message')

    def path_to_device(self, path):
        return self.bus.get_object('org.freedesktop.systemd1.Manager', path)

    def device_added(self, *args, **kwds):
        path = kwds['path']
        print 'Added', path
        properties = self.path_to_device(path).GetAllProperties()
        print properties.get('info.category')
        if properties.get('info.category') == u'volume':
            label, dev = properties.get('volume.label'), properties.get('block.device')
            print 'Mounting %s on /mnt/%s' %(dev, label)
            ## os.system('mount %s /mnt/%s' %(dev, label))

    def device_removed(self, *args, **kwds):
        path = kwds['path']
        print 'Removed', path

    def print_event(*args, **sender):
        print "got signal from:"
        print sender

if __name__ == '__main__':
    DBusGMainLoop(set_as_default=True)

    m = DeviceManager()

    mainloop = GLib.MainLoop()
    try:
        mainloop.run()
    except KeyboardInterrupt:
        mainloop.quit()
        print 'Exiting...'
        sys.exit(0)