Lock screen doesn't emit ActiveChanged signal

tl;dr; Unity doesn't emit the ActiveChanged signal when the screen is locked/unlocked

Long version:

unity-settings-daemon's automount plugin has code to detect whether the screen is locked or not before automatically mounting a volume. This prevents someone from inserting a USB thumb drive when the screen is locked and exploiting a possible nautilus thumbnailer vulnerability. (See bug #714958 for original implementation details.)

In Ubuntu 14.04, this code no longer works. Inserting a USB thumb drive while the screen is locked results in a Nautilus window opening underneath the lock screen, and the contents of the USB thumb drive being read.

Since the screen lock got switched to Unity in Ubuntu 14.04, Unity no longer emits the org.gnome.ScreenSaver ActiveChanged signal when the screen gets locked or unlocked.

To test:

1- in terminal, type:
dbus-monitor "type='signal',sender='org.gnome.ScreenSaver',interface='org.gnome.ScreenSaver'"
2- Lock the screen
3- Unlock the screen
4- Notice that no signal was received

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: unity 7.3.2+15.04.20150330-0ubuntu1
ProcVersionSignature: Ubuntu 3.19.0-10.10-generic 3.19.2
Uname: Linux 3.19.0-10-generic x86_64
ApportVersion: 2.17-0ubuntu1
Architecture: amd64
CompizPlugins: No value set for `/apps/compiz-1/general/screen0/options/active_plugins'
CurrentDesktop: Unity
Date: Tue Mar 31 15:15:48 2015
InstallationDate: Installed on 2013-11-26 (489 days ago)
InstallationMedia: Ubuntu 13.10 "Saucy Salamander" - Release amd64 (20131016.1)
SourcePackage: unity
UpgradeStatus: Upgraded to vivid on 2015-03-07 (24 days ago)