Changed HsumBootUserInitializer so it promotes an admin to main user.

Prior to this change, when a device changed from not requiring a main
user to requiring one, it would create a new user.

Test: atest HsumBootUserInitializerTest
Test: atest FrameworksMockingServicesTests --test-filter=".*UserManagerServiceMockedTest#testSetMainUser.*"

Bug: 402486365
Flag: android.multiuser.create_initial_user

Change-Id: I6807d6ab12a16cc5a892d1bdca91a4fd3b761f3e