Hide "Changes made by your organization's admin" when empty

The admin of a manged device can take a number actions that will be
listed in the "Changes made by your organization's admin" section of
Settings. If the admin has not taken any such actions, the section
will be empty and should be hidden. This is accomplished by having a
PreferenceController for the section that observes the state of the
PreferenceControllers inside it.

Bug: 35912953
Test: m RunSettingsRoboTests

Change-Id: Ia95754493ee6c5a19b4aa9731fd56fd558e61849