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