policykit auth dialog displayed on top of the screen lock

[Jean-Baptiste Lallement]

core 16-2.28.1 from candidate

Test Case
1. With gnome-software, remove a previously installed snap
2. When the polkit auth dialog shows up, wait until the screen lock goes off
3. Unlock the screen

Actual result
The polkit dialog is displayed on top of the screen lock which cannot be unlocked

ProblemType: Bug
DistroRelease: Ubuntu 17.10
Package: policykit-1-gnome 0.105-6ubuntu2
ProcVersionSignature: Ubuntu 4.13.0-12.13-generic 4.13.3
Uname: Linux 4.13.0-12-generic x86_64
ApportVersion: 2.20.7-0ubuntu2
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Fri Oct 6 12:44:41 2017
InstallationDate: Installed on 2014-07-23 (1170 days ago)
InstallationMedia: Ubuntu 14.04.1 LTS "Trusty Tahr" - Release amd64 (20140722.2)
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=fr_FR.UTF-8
 SHELL=/bin/bash
SourcePackage: policykit-1-gnome
UpgradeStatus: Upgraded to artful on 2017-06-13 (114 days ago)