Don't backup/restore screen brightness

We no longer want to backup and restore screen brightness as it could
leave the new device in an unusable state and doesn't make sense
cross-device.

Test: 1) atest SettingsBackupTest
2) atest SettingsValidatorsTest
3) Manual:
- Old backup set that has screen brightness does not restore with change
- Screen brightness does not backup or restore with change
Bug: 77583401

Change-Id: I8a6d950717e6aeb9bf6773c14708ee70069f9df0