Created config_designateMainUser.

Currently on HSUM devices, the Main User is created on boot when
config_isMainUserPermanentAdmin is true, but the main purpose of that
config is to make sure the Main User cannot be removed.

This CL "moves" the Main User creation to config_designateMainUser,
which in turn can be overridden by the system property
fw.designate_main_user_on_boot (for development purposes).

Test: adb shell setprop fw.designate_main_user_on_boot false && adb shell dumpsys system_server_dumper --name SystemServer
Bug: 402486365
Flag: EXEMPT refactor

Change-Id: If894e769e986b90dce89ec7c69db95badeecfccc