core: don't emit double PropertiesChanged signal for new active connections

core: don't emit double PropertiesChanged signal for new active connections

When porting to GDBus property change notifications were converted from a
hash table to a GVariantBuilder.  GVariantBuilder doesn't care about
duplicated properties in the dict so each g_object_notify() will add
an additional item with possibly different values:

signal time=1458571005.592811 sender=:1.10 -> destination=(null destination) serial=64451 path=/org/freedesktop/NetworkManager; interface=org.freedesktop.NetworkManager; member=PropertiesChanged
   array [
      dict entry(
         string "ActiveConnections"
         variant             array [
               object path "/org/freedesktop/NetworkManager/ActiveConnection/19"
               object path "/org/freedesktop/NetworkManager/ActiveConnection/18"
               object path "/org/freedesktop/NetworkManager/ActiveConnection/15"
               object path "/org/freedesktop/NetworkManager/ActiveConnection/0"
            ]
      )
      dict entry(
         string "ActiveConnections"
         variant             array [
               object path "/org/freedesktop/NetworkManager/ActiveConnection/24"
               object path "/org/freedesktop/NetworkManager/ActiveConnection/19"
               object path "/org/freedesktop/NetworkManager/ActiveConnection/18"
               object path "/org/freedesktop/NetworkManager/ActiveConnection/15"
               object path "/org/freedesktop/NetworkManager/ActiveConnection/0"
            ]
      )
   ]

Fix that by not emitting notify events for the manager's ActiveConnections
property until the property has actually been updated in active_connection_add().
The unexport also isn't required for VPN connections since it will get
unexported when it's disposed after _internal_activation_failed() gets called.