Fix Connected devices page UI not update after paired

This CL before, onChanged(null) will cause PreferenceGroup
didn't update UI in ConnectedDeviceDashboardFragment.

This CL remove onChanged(null) and set unvisible of SliceView
in the constructor.

Bug: 142220754
Test: make -j42 RunSettingsRoboTests
Change-Id: Ib20db373430589c5c5efbe8a4c866b7fae4f3399

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

@@ -46,6 +46,7 @@ public class SlicePreference extends LayoutPreference {
         mSliceView = findViewById(R.id.slice_view);
         mSliceView.setShowTitleItems(true);
         mSliceView.setScrollable(false);
+        mSliceView.setVisibility(View.GONE);
     }
 
     public void onSliceUpdated(Slice slice) {