wifi: expose the LastScan property

This is the time when the last Wi-Fi scan finished. It will help clients
decide whether the AP list is fresh enough.

tools/test-networkmanager-service.py

@@ -908,6 +908,7 @@ PRP_WIFI_BITRATE = "Bitrate"
 PRP_WIFI_ACCESS_POINTS = "AccessPoints"
 PRP_WIFI_ACTIVE_ACCESS_POINT = "ActiveAccessPoint"
 PRP_WIFI_WIRELESS_CAPABILITIES = "WirelessCapabilities"
+PRP_WIFI_LAST_SCAN = "LastScan"
 
 class WifiDevice(Device):
     def __init__(self, iface, mac = None, ident = None):
@@ -926,6 +927,7 @@ class WifiDevice(Device):
             PRP_WIFI_WIRELESS_CAPABILITIES: dbus.UInt32(0xFF),
             PRP_WIFI_ACCESS_POINTS:         ExportedObj.to_path_array(self.aps),
             PRP_WIFI_ACTIVE_ACCESS_POINT:   ExportedObj.to_path(None),
+            PRP_WIFI_LAST_SCAN:             dbus.Int32(0x70000000),
         }
 
         self.dbus_interface_add(IFACE_WIFI, props, WifiDevice.PropertiesChanged)