unable to set background on snappy

[Jonas G. Drange]

Reproduce:
U8 session snap -> System Settings -> Background -> Choose background and set

What happens:
Doesn't change

What should happen:
The background should change

Log:
file:///snap/unity8-session/x1/usr/share/ubuntu/settings/system/qml-plugins/background/Preview.qml:40: TypeError: Cannot call method 'hasOwnProperty' of null
file:///snap/unity8-session/x1/usr/share/ubuntu/settings/system/qml-plugins/background/Preview.qml:35: TypeError: Cannot read property '__styleInstance' of null
file:///snap/unity8-session/x1/usr/share/ubuntu/settings/system/qml-plugins/background/Preview.qml:123: TypeError: Cannot read property of null
org.freedesktop.DBus.Error.ServiceUnknown: The name com.canonical.usensord was not provided by any .service files
Could not call AccountsService method "SetBackgroundFile" for object "/org/freedesktop/Accounts/User1001" with argument QVariant(QString, "/var/lib/lightdm-data/snappy-test/ubuntu-system-settings/Pictures/System/Black_hole_by_Marek_Koteluk.jpg") : "file '/var/lib/lightdm-data/snappy-test/ubuntu-system-setti
ngs/Pictures/System/Black_hole_by_Marek_Koteluk.jpg' is not a regular file"
file:///snap/unity8-session/x1/usr/share/ubuntu/settings/system/qml-plugins/background/Preview.qml:45: TypeError: Cannot call method 'hasOwnProperty' of null

[Jonas G. Drange (jonas-drange)]

/var/lib/lightdm-data/ is XDG_GREETER_DATA_DIR, but we can no longer write to this. I suggest moving it to $SNAP_COMMON, but I have no idea what this really entails.