Avoid invoking permission change for VPN networks.

Setting permissions is only supported for Physical networks
and setting it for virtual networks leads to exception in
SystemServer. Add an addition check to verify if the
NetworkAgentInfo is VPN (Virtual Network) and avoid invoking
set-permission.

Test:
- Builds, Boots
- No exceptions on Enabling VPN
- VPN connection successful.

CRs-Fixed: 2409643
Change-Id: I4881db4a4d59635165cb5936858bef5d5d170b75