Start 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 takes the following steps toward fixing this:
* Pass the list of PreferenceControllers to the search indexer
* Make the first two PrefrenceControllers set isAvailable() correctly

There are more disclosures to update, but the difficult work is done
as all others will follow the same pattern.

Bug: 32692748
Test: m RunSettingsRoboTests

Change-Id: I7d3e248b80abe72b79fce7afa11f28a822de6986