wallpapers/potd: update wallpapers after network is connected
Usually after waking up from sleep and when the engine receives
PrepareForSleep
from org.freedesktop.login1.Manager, the network
is still disconnected, so the engine needs to update wallpapers after
the network is connected.
In case the network is connected before PrepareForSleep
, the loading
condition will block duplicate update requests.
BUG: 457361 FIXED-IN: 5.26