Fallback to Cellular if WiFi fails to validate

Previously, once a network validated, for the purposes of comparing networks
to select the default network, we always considered it validated.
With this change if a network later fails to validate, we'll take this latest
validation result into account.  This means if WiFi and cellular are up
(e.g. if we recently switched from cellular->WiFi, and cellular is now
lingering) and both are validated, but for some reason WiFi fails a validation,
cellular will become the default network connection.

Bug:20896761
Change-Id: I858aa10c1aaec5cd9032067f960963409107bdb1