Add server information for pptp vpn

The pptp vpn set up in ip-up-vpn.c does not provide an environment
variable for the remote server so we cannot setup a correct throw route
to the VPN endpoint. Change the LegacyVpnRunner class to include the
remote server information in the class so it can use it to bring up the
vpn and set the throw rule correctly.

Bug: 131386220
Test: Build
Change-Id: Ia6b00dd88f8f31823647c56c20b8c7edd20280f7