Changed setMainUser() so it checks for create_initial_user flag.

Ideally it should only check for demote_main_user, but then an admin
user couldn't be promoted when rolling back that flag.

Test:  atest FrameworksMockingServicesTests --test-filter=".*UserManagerServiceMockedTest.*testSetMainUser.*"
Fixes: 441582871
Flag: android.multiuser.create_initial_user

Change-Id: If719068a1f5ab3acd1eb130abbe20d716502f870