Update icons and strings for always-on VPN info

- Replaces the realistic outline of a key with the key icon used in
  other places, eg. status bar icon

- Updates string "tap to configure" to "tap to set up" and adds another
  string "always-on vpn disconnected" which is more neutral than
  "always-on VPN failed".

Bug: 29123115
Change-Id: I4685758b682dbc947423d4ec81891506a7dc826e