Finish removing N/A DO disclosures from search index

DO disclosures referring to actions that the admin did not actually
take are hidden in the UI, but still show up in the search index. This
CL switches the remaining PreferenceControllers from setting visibility
to implementing isAvailable() instead.

Bug: 32692748
Test: m RunSettingsRoboTests

Change-Id: I54360698f28b549b18cdc230e3b9087cf4e9ff4a