Add summary text to Display->Brightness level.

Get the current brightness level from settings provider and set it to
the brightness preference summary.

Change-Id: Ieb4fbc132ec120cf839548f64e9c5f5b52b2f9c2
Fix: 36650108
Test: make RunSettingsRoboTests

src/com/android/settings/display/BrightnessLevelPreferenceController.java

@@ -0,0 +1,49 @@
+/*
+ * Copyright (C) 2017 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.display;
+
+import static android.provider.Settings.System.SCREEN_BRIGHTNESS;
+
+import android.content.Context;
+import android.provider.Settings;
+import android.support.v7.preference.Preference;
+import com.android.settings.core.PreferenceController;
+import java.text.NumberFormat;
+
+public class BrightnessLevelPreferenceController extends PreferenceController {
+
+    private static final String KEY_BRIGHTNESS = "brightness";
+
+    public BrightnessLevelPreferenceController(Context context) {
+        super(context);
+    }
+
+    @Override
+    public boolean isAvailable() {
+        return true;
+    }
+
+    @Override
+    public String getPreferenceKey() {
+        return KEY_BRIGHTNESS;
+    }
+
+    @Override
+    public void updateState(Preference preference) {
+        final double brightness = Settings.System.getInt(mContext.getContentResolver(),
+            SCREEN_BRIGHTNESS, 0);
+        preference.setSummary(NumberFormat.getPercentInstance().format(brightness / 255));
+    }
+
+}