Bug 1441822 - Add upower-observe to plugs in snap r=jlorenzo

MozReview-Commit-ID: FqJ9lC9PwRF

--HG--
extra : rebase_source : 4d2a11cba2ca059738e1f7a385732e8611154b8d

taskcluster/docker/firefox-snap/snapcraft.yaml.in

@@ -26,6 +26,7 @@ apps:
       - pulseaudio
       - screen-inhibit-control
       - unity7
+      - upower-observe
       - x11
 
 plugs: