Various createUsers don't sometimes return null

Previously, UserManagerService.createUserInternalUncheckedNoTracing
would usually throw on failure, but could instead return null in certain
circumstances (namely, bad input).

This lead to ambiguity over whether the various createUser methods were
able to return null.

Here, we throw instead in these previously-null situations. This way,
all of the createUser methods are @NonNull, unless they specifically
catch errors and return null instead. The methods are now all
self-consistent.

In particular, this fixes the previously not-always-correct annotation
of @NonNull that is on UserManager.preCreateUser.

Bug: 219942927
Test: atest com.android.server.pm.UserManagerTest
Test: atest com.android.server.pm.UserManagerServiceTest
Test: atest android.multiuser.cts.UserManagerTest
Change-Id: I7a63b4e684d03b4d2ef76046d0cf277ba2d5744d