Boot to headless system user if config_bootToHeadlessSystemUser is true

Only do this under HSUM.

Bug: 350058102
Flag: EXEMPT new code protected by new config
Test: Set the new config to true in a build, boot up, and verify the result from "adb shell ps -e -o USER,UID,NAME | grep systemui" is user 0 (UID is lower than 1000000).
Test: atest FrameworksMockingServicesTests:com.android.server.pm.UserManagerServiceTest
Change-Id: I7cf36a9900cfe01383de9c935781fb00fe80f90c