Fix isActiveNetworkMetered for VPNs.

This change is basically a revert of http://ag/3580901.
It was made because previously VPN capabilities did not use to update
based on its underlying networks. That is no longer the case anymore.

This was previously returning meteredness on the basis of VPN's first
underlying network which is incorrect in cases such as VPN using
multiple underlying networks, or VPN that has explicitly marked itself
as metered via VpnService.Builder#setMetered API.

Bug: 123936838
Test: atest FrameworksNetTests
Change-Id: Ia54b8570fbad4a638a6d43a95e0271c6baf66685