Add a new boot strategy for headless system user mode

The new boot strategy is to boot to the system user when the device
is provisioned, and to boot to the first switchable full user when
the device is not provisioned. This is to ensure that the system user
is always available for system services, even when the device is not
provisioned.

Context: see DD in the bug

Bug: 374926694
Change-Id: Ia0a30028be2ac9575a2dba7a01dde095cdcea092
Flag: EXEMPT new code protected by new config
Test: manually build and deploy in DUT and test both cases
Test: atest FrameworksMockingServicesTests:com.android.server.pm.UserManagerServiceTest