Settings: Use public API for mac randomization support

Also, don't directly reference the default tether ssid overlay. This info
should always be retrieved from wifiManager.
This ensures that we don't use wifi overlays in settings.

Bug: 143464763
Test: atest com.android.server.wifi
Change-Id: I893a4ca0f4243e1eb6f9c8bef810d0b4a493fb88