Add condition provider access to settings.

Pull NotificationAccessSettings out into a reusable base class
and reuse all of it for a new condition providers settings page.

As with notification listeners, the entire section is hidden if
no apps provide conditions.

Change-Id: Ib5273f3362e392d99647347c23a514e4590f7ac9

res/layout/managed_service_settings.xml

@@ -31,7 +31,6 @@
                 android:layout_width="match_parent"
                 android:layout_height="match_parent"
                 android:gravity="center"
-                android:text="@string/no_notification_listeners"
                 android:textAppearance="?android:attr/textAppearanceMedium" />
     </FrameLayout>
 </LinearLayout>