Update longLivedTcpConnectionsExpensive value in VpnTransportInfo

The longLivedTcpConnectionsExpensive value relies on the keepalive
value, so the Vpn should send network capabilities update to renew
the VpnTransportInfo. The applications that may bypass VPN based
on the value could update with it accordingly.

The VpnTransportInfo should be recreated when
1. The NeworkAgent is created
2. The underlying network of the VPN network is updated
3. The carrierconfig is changed

The NetworkCapabilities may be updated more frequently than these
3 events. But the others does not affect the keepalive value so
it's fine to used the TransportInfo from existing
NetworkCapabilities.

Bug: 273422889
Test: atest FrameworksNetTests
Change-Id: Ia1a124fc06d2d882eb154f24a75493ad0a308a78