Fix title of PRIVATE_DNS_BROKEN notification when connecting VPN

NetworkNotificationManager will only get the first transport
type from the NetworkCapabilities of network, and if the device
connects to a VPN and its underlying network is wifi, then the
first finding transport type will be TRANSPORT_WIFI. So, if the
private DNS is broken when device connected to VPN,
NetworkNotificationManager will try to get the SSID for the
title of notification but failed. For this kind of case, the
title of PRIVATE_DNS_BROKEN notification will show
"null has no internet access".

Bug: 143340533
Test: 1. Build pass.
      2. Connect to VPN and let private DNS to be broken, check
      title of PRIVATE_DNS_BROKEN notification.
      3. atest FrameworksNetTests

Change-Id: I1ed018cc8774d4fce4b94854f8e8703a28818463