Backup SystemUI as system user

Ensure the backup runs as the system user, to match
SystemUI. Subsequent work for SystemUI will ensure that the data is
corrected back up for the target user ID, which may be different than
the system user. In the future, this will always be true.

Bug: 261168533
Test: manual (backup and restore from settings)
Change-Id: I38631c15f092355fe8f7b0afd1a69d3917685fdf