Check for WiFi controller for summary population.

WiFiController might return not available, so we should only surface
"Wi-Fi" as a summary entry if it's available in the settings page.

This also checks for empty strings in case they are declared as such in
the XML (since XML can be device/product overlay'd), to avoid situations
such as ", data usage".

Bug: 111398942
Test: Manual check, and I see Wi-Fi no longer visible for cases where
Wifi controller isn't available.
Test: make RunSettingsRoboTests
Change-Id: I24736a6240c50e2ea38ce424fbac2316721290d8