[CrOS Networking] Fix system tray network scanning UI.

The network UI in the system tray stopped displaying the scanning
animation in M-74 due to a regression which removed the
DevicePropertiesUpdated() handler from TrayNetworkStateObserver. This CL
re-adds this observer, which causes the tray to display the scanning
animation again.

Bug: 935723
Change-Id: I89b8531558a662f976b005094172f518638fa262
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1516884
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Auto-Submit: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#639759}