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
This CL allows the user to see on the Enterprise Privacy Settings
page whether the admin set a global HTTP proxy.
Test: make RunSettingsRoboTests
Bug: 32692748
Change-Id: I3c7c46f806f39c90425fd8e098a749f3cc1e9278