Fix duplicate SSID entries in the WifiSettings UI

The root cause is likely our 'no networks' problem; it caused people
to add new (and duplicate) entries to their list of networks; which
are haunting us now.

This change looks up the dupes, and discards them, even if they came
wifi supplicant.

Bug: 10633813
Change-Id: I656b6bf7240de6763bf84b5b406c50444af6dd48