Change flag used on HsumBootUserInitializer.designateMainUserOnBoot()

Initially it was using create_initial_user, but that flag is already
ramped on main, so it's better to use demote_main_user instead:

$ adb shell aflags list | egrep '(create_initial_user|demote_main_user)'
android.multiuser.create_initial_user enabled  - default read-write system
android.multiuser.demote_main_user    disabled - default read-write system

Bug: 402486365
Test: atest HsumBootUserInitializerDesignateMainUserOnBootTest
Flag: android.multiuser.demote_main_user

Change-Id: I866d20b9356976e922168e41968a1a8dd56189fb