Fix VPN connected logic in internet settings

This CL:
* Fixes the logic that updates 'deviceStates' to avoid an incorrect
  intermediate state.
* Adds a defaultNetwork property allowing the details page to check
  that a default network exists to enable 'connect' for VPN.
* Updates the enabled logic for the connect button for VPN

BUG=521040

Review URL: https://codereview.chromium.org/1403773002

Cr-Commit-Position: refs/heads/master@{#356324}