Support WiFi only device at runtime.

To date WiFi only devices were defined by the list of networkAttributes
in config.xml overriden in on a per-device basis. This change is the
simplest change needed to determine this at runtime and therefore allowing
a single build to support the two different configurations.

Bug: 8562845
Change-Id: I34de5c6accc718b199c13815537de1debfe3dc91