Implement support for bypassable VPNs.

Bypassable VPNs grab all traffic by default (just like secure VPNs), but:
+ They allow all apps to choose other networks using the multinetwork APIs.
  If these other networks are insecure ("untrusted"), they will enforce that the
  app holds the necessary permissions, such as CHANGE_NETWORK_STATE.
+ They support consistent routing. If an app has an existing connection over
  some other network when the bypassable VPN comes up, it's not interrupted.

Bug: 15347374
Change-Id: Iaee9c6f6fa8103215738570d2b65d3fcf10343f3