Allow "None" to be selected as a Network rating provider.

Updated the picker to always provide "None" as an option. The
preference is only disabled if there a no valid scorers installed.
Existing tests updated and new tests added.

Bug: 37722313
Test: make RunSettingsRoboTests -j40
Test: manual on device clicking.
Change-Id: Ie6fd9f29e0ea3c64845dd98ce1d117013f50c86b