Add UiBlocker for DashboardFragment

If DashboardFragment detects blocker controllers, it won't display
until:
1. All blocking controllers finish bg works.
2. Timeout

This CL adds UiBlockerController to control this behavior and
BlockingSlicePreferenceController as an example.

Bug: 120803703
Test: atest SettingsUnitTests
Change-Id: I66fc194776d46ee49b3ec7685f3167834e673ba2

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

@@ -0,0 +1,43 @@
+/*
+ * Copyright (C) 2018 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.android.settings.slices;
+
+import android.content.Context;
+
+import androidx.slice.Slice;
+
+import com.android.settings.core.BasePreferenceController;
+
+/**
+ * The blocking slice preference controller. It will make whole page invisible for a certain time
+ * until {@link Slice} is fully loaded.
+ */
+public class BlockingSlicePrefController extends SlicePreferenceController implements
+        BasePreferenceController.UiBlocker {
+
+    public BlockingSlicePrefController(Context context, String preferenceKey) {
+        super(context, preferenceKey);
+    }
+
+    @Override
+    public void onChanged(Slice slice) {
+        super.onChanged(slice);
+        if (mUiBlockListener != null) {
+            mUiBlockListener.onBlockerWorkFinished(this);
+        }
+    }
+}