Enable auto-brightness adjustment feature with system prop.

This feature is experimental and disabled by default.

Change-Id: I183534370da7613557e4d1535f556873f8a763df

Author: Jeff Brown

core/java/android/os/PowerManager.java

@@ -330,6 +330,15 @@ public int getDefaultScreenBrightnessSetting() {
                 com.android.internal.R.integer.config_screenBrightnessSettingDefault);
     }
 
+    /**
+     * Returns true if the screen auto-brightness adjustment setting should
+     * be available in the UI.  This setting is experimental and disabled by default.
+     * @hide
+     */
+    public static boolean useScreenAutoBrightnessAdjustmentFeature() {
+        return SystemProperties.getBoolean("persist.power.useautobrightadj", false);
+    }
+
     /**
      * Creates a new wake lock with the specified level and flags.
      * <p>

services/java/com/android/server/power/DisplayPowerController.java

@@ -83,7 +83,8 @@ final class DisplayPowerController {
     private static final boolean USE_ELECTRON_BEAM_ON_ANIMATION = false;
 
     // If true, enables the use of the screen auto-brightness adjustment setting.
-    private static final boolean USE_SCREEN_AUTO_BRIGHTNESS_ADJUSTMENT = false;
+    private static final boolean USE_SCREEN_AUTO_BRIGHTNESS_ADJUSTMENT =
+            PowerManager.useScreenAutoBrightnessAdjustmentFeature();
 
     // The maximum range of gamma adjustment possible using the screen
     // auto-brightness adjustment setting.