Listen to brigtness updates to update brightness summary.

- add content observer for brightness relates changes, and update the
brightness preference summary when changes occur.
- also check for VR mode and auto brightness mode when reading the
current brightness value.

Bug: 37227609
Test: make RunSettingsRoboTests
Change-Id: I895d251e1dc12cb114ef00328eb8aa0075d77ab1

src/com/android/settings/DisplaySettings.java

@@ -104,7 +104,7 @@ public class DisplaySettings extends DashboardFragment {
         controllers.add(new VrDisplayPreferenceController(context));
         controllers.add(new WallpaperPreferenceController(context));
         controllers.add(new ThemePreferenceController(context));
-        controllers.add(new BrightnessLevelPreferenceController(context));
+        controllers.add(new BrightnessLevelPreferenceController(context, lifecycle));
         return controllers;
     }