Update SlicePreferenceController

Make sure setSliceUri is invoked even when it is null, where we create
SliceLiveData.

Then in lifecycle, if LiveData is not null, we will register observer.

Bug: 120803703
Test: Manual

Change-Id: I56caf2abf04c4a251ebafa5deb599d44b1c7fe92

src/com/android/settings/slices/SlicePreferenceController.java

@@ -82,6 +82,8 @@ public class SlicePreferenceController extends BasePreferenceController implemen
 
     @Override
     public void onChanged(Slice slice) {
-        mSlicePreference.onSliceUpdated(slice);
+        if (slice != null) {
+            mSlicePreference.onSliceUpdated(slice);
+        }
     }
 }