Update Settings UI for IKEv2/IPsec VPNs

This CL updates Settings for IKEv2/IPsec Platform VPN profiles.

Platform VPN profiles currently configure DNS and routes based on the
configuration received from the server. As such, these parameters are
not required to start an always-on VPN.

Similarly, a numeric server address is not required, as the IKEv2
library will do the DNS resolution based on the current Network. This
has the nice property of allowing the VPN to run with IPv4 or IPv6 outer
addresses (as opposed to LegacyVpn, which runs only in IPv4)

Lastly, this always allows configuration of the IKEv2 local identifier,
whether MSCHAPv2, RSA or PSK authentication is used.

Bug: 148991741
Test: Compiles, manually tested.
Change-Id: Ib1049fdc602e349bb0d24de536767a6e15adf194