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

tests/robotests/src/com/android/settings/dream/CurrentDreamPreferenceControllerTest.java

@@ -56,7 +56,7 @@ public class CurrentDreamPreferenceControllerTest {
     public void setup() {
         MockitoAnnotations.initMocks(this);
 
-        mController = new CurrentDreamPreferenceController(mContext);
+        mController = new CurrentDreamPreferenceController(mContext, "test");
         ReflectionHelpers.setField(mController, "mBackend", mBackend);
     }