Changed HsumBootUserInitializer to create a non-main initial user.

Currently, HBUI only creates a main user when the device is
configured to require one; otherwise, it assumes the initial user
will be created by something (like CarService).

This CL adds a 3rd option: create a non-main initial user.

Fix: 409650316
Test: atest FrameworksMockingServicesTests:HsumBootUserInitializerTest
Flag: android.multiuser.create_initial_user

Change-Id: I384fdf0e80802dc9aff598697b0d9ba17140ea15