Improve BT slice card loading performance

The bottleneck is getting LocalBluetoothManager the first time.
1. Initialize LocalBluetoothManager earlier and asynchronously.
2. Don't block in slice's constructor and getSlice().
  - Initialize the bt updaters until the manager is ready.
  - Just show a header if the manager is not ready yet.

Fixes: 157702021
Test: robotest
Change-Id: I427df55f259b45ba4c37557b22e09dcc24079e93

src/com/android/settings/homepage/contextualcards/ContextualCardsFragment.java

@@ -37,6 +37,7 @@ import androidx.recyclerview.widget.ItemTouchHelper;
 
 import com.android.settings.R;
 import com.android.settings.core.InstrumentedFragment;
+import com.android.settings.homepage.contextualcards.slices.BluetoothUpdateWorker;
 import com.android.settings.homepage.contextualcards.slices.SwipeDismissalDelegate;
 import com.android.settings.overlay.FeatureFactory;
 import com.android.settings.wifi.slice.ContextualWifiScanWorker;
@@ -67,6 +68,7 @@ public class ContextualCardsFragment extends InstrumentedFragment implements
         final Context context = getContext();
         if (savedInstanceState == null) {
             FeatureFactory.getFactory(context).getSlicesFeatureProvider().newUiSession();
+            BluetoothUpdateWorker.initLocalBtManager(getContext());
         }
         mContextualCardManager = new ContextualCardManager(context, getSettingsLifecycle(),
                 savedInstanceState);