Back up VR screen brightness setting.

'adb help backup' to see how to do backup/restore to a file.

Verified value did not backup prior to change and that it did afterward.

Bug: 34437367
Testing: Set to 30%, do backup, set to 100%, do restore, should be 30%.

Change-Id: I43ce31bf50bc4608e8ce9b87ea0bdb83bac52e61

core/java/android/provider/Settings.java

@@ -3866,6 +3866,7 @@ public final class Settings {
             SCREEN_BRIGHTNESS,
             SCREEN_BRIGHTNESS_MODE,
             SCREEN_AUTO_BRIGHTNESS_ADJ,
+            SCREEN_BRIGHTNESS_FOR_VR,
             VIBRATE_INPUT_DEVICES,
             MODE_RINGER_STREAMS_AFFECTED,
             TEXT_AUTO_REPLACE,