Fix WiFi toggle issue of Internet Settings

- Because b/209906859 needs to add WiFi service check for WiFi-less
pixel project.

- But mWifiManager has not been initialized before the
addWifiSwitchPreferenceController() function, which will cause the
controller to not handle the WiFi switching UI.

- Add a hasWifiManager() function to initialize the WiFi manager and
handle related functions.

Bug: 212211915
Test: manual test
make RunSettingsRoboTests ROBOTEST_FILTER=NetworkProviderSettingsTest

Change-Id: Idc0d4cdc31403f0ab9a4eb787e4c70be605f7ddb