Populate ipsecSecret field in VpnProfile

This commit fixes IKEv2 RSA VPN could not be an always-on VPN
issue to populate ipsecSecret field from "IPsec user certificate".

Fields ipsecUserCert and ipsecSecret are required for a valid
VpnProfile. The ipsecSecret field in VpnProfile should be
populated from "IPsec user certificate" field. It's the matching
private key.

Bug: 263610164
Bug: 230727391
Test: Verify if IKEv2 RSA VPN can setup successfully and set as
      as always on VPN
Change-Id: I605b35fff9e399ee7b82d8441137984992ab2907