Update strings in main VpnPreference.

If there is an insecure VPN, don't display the name of the vpn in the
summary. Instead, display "Not secure" if there is only 1 total vpn
available, and display the number of insecure VPNs + " not secure."

Screenshot: https://screenshot.googleplex.com/9Bxn7frDm7aVHtG
Test: manual test (because the junit test for the controller has not yet
been cherrypicked into sc-dev)
Bug: 176821216
Change-Id: Ie454d9605effd5208457e8c91b08cc382d4992af