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
784ca66669