Fix crash when starting daydream setting with wrong context

The context must be a UI context. This is a new requirement in P.

(Also converted the CurrentDream controller to be slice compatible)

Change-Id: I28874f296da617f4cedf6706b0663b76671f1780
Fixes: 111375261
Test: robotests

src/com/android/settings/dream/DreamSettings.java

@@ -132,7 +132,6 @@ public class DreamSettings extends DashboardFragment {
 
     private static List<AbstractPreferenceController> buildPreferenceControllers(Context context) {
         List<AbstractPreferenceController> controllers = new ArrayList<>();
-        controllers.add(new CurrentDreamPreferenceController(context));
         controllers.add(new WhenToDreamPreferenceController(context));
         controllers.add(new StartNowPreferenceController(context));
         return controllers;