Add ability to show/hide Wi-Fi settings, and all related sub-settings.

This adds a new boolean flag, config_show_wifi_settings, which when set
to false will hide the "Wi-Fi" page in network & internet (there is no
key value associated with this preference page). It will also hide
nested pages from search results, such as Wi-Fi preferences.

Bug: 62379554
Test: make RunSettingsRoboTests
ROBOTEST_FILTER=ConfigureWifiSettingsTest
and
make RunSettingsRoboTests
ROBOTEST_FILTER=WifiSettingsTest
and
make RunSettingsRoboTests
ROBOTEST_FILTER=WifiMasterSwitchPreferenceControllerTest
all pass.

Change-Id: Iedc6bc272e0cb2b561c6bf25968e70f355aaaa8e