Change headless system user's name

This CL changes the default user name for HSU to "System User".
This value can be overridden by overlay.

Bug: 410929293
Bug: 407568535
Flag: android.multiuser.logout_user_api
Test: Manual test. adb shell am logout-user 10
Test: atest FrameworksMockingServicesTests:UserManagerServiceMockedTest

Change-Id: I375191126d4300ff036327484d5fd03d9b306786