Expose a SystemApi method to prepare a VPN without consent.

This is NOT designed to be called normally. Most apps (even
system-privileged ones) should request user consent before launching a
VPN. However, it is needed to support flows where consent can be
obtained through other means external to the VPN flow itself.

The API requires a system-privileged permission, CONTROL_VPN.

Bug: 18327583
Change-Id: I1bcdcf0fb5707faeb861ec4535e7ccffea369ae7