Maximum user limits, handled consistently

Makes the various methods related to max users (what is the max, have we
reached it, how many more are allowed, actual creation procedure)
consistent between each other, and across user types, with a single
underlying source-of-truth method (getRemainingCreatableUserCount).

config_multiuserMaximumUsers now serves as the actual device max,
without making the situation exceptional for certain users. With both
flags enabled (see below), this even includes Guest users. The goal is
that OEMs can set this value to be very high (based only on storage, not
UX considerations), and to instead use config_user_types.max-allowed to
achieve fine-grained specification of maxima per user-type (based on
UX). This will allow better support for multiple profile types, without
compromising the full user experience.

Some changes:
* config_multiuserMaximumUsers - no longer has a bunch of exceptions,
  and now serves as the actual device max
* canAddMoreUsers - now is correct for all user types, including demo,
  managed profile, and private profiles
* getRemainingCreatableUserCount - simplified logic by amalgamating
  userType considerations into getMaxSupportedUsersOfType. Now takes
  into account all feature support too (including for private profiles).
  Serves as ultimate source-of-truth!
* getRemainingCreatableProfileCount - simplified and now calls
  getRemainingCreatableUserCount
* createUser - is now consistent with the other methods
* canAddPrivateProfile - no change to behaviour, but is now integrated
  into the other methods properly

Two flags are used to gate these changes:
* android.multiuser.consistentMaxUsers
  Mostly just a refactor, to make a single source-of-truth for how to
calculate user limits. There are technically some effects on Demo user
creation, since it no longer inconsistently gets a partial exemption
from the limits, but that doesn't really matter since Demo users don't
get used that way in practice.

* android.multiuser.consistentMaxUsersIncludingGuest
  Makes it so that Guest users are now subject to the overall max, just
like all other user types. This would have the effect of decreasing the
overall number of users that can be created, since Guests now count.
This should be balanced by changing the config_multiuserMaximumUsers
value, but since this requires an active change, we guard this with a
separate flag.

Note: preCreated users no longer are allowed to exceed their user type
max. This isn't particularly desired, but precreated users are
completely deprecated (see bugs 253528462 and 282987119), so there isn't
any point added to the complexity to exempt them, and one could argue
either way. No need to flag gate this.

Flag: android.multiuser.consistent_max_users
Flag: android.multiuser.consistent_max_users_including_guests
Flag: android.multiuser.max_users_in_car_is_for_secondary
Bug: 394178333
Test: atest FrameworksServicesTests:com.android.server.pm.UserManagerTest#testAddTooManyUsers_Secondary
Test: atest FrameworksServicesTests:com.android.server.pm.UserManagerTest#testAddTooManyUsers_Guest
Test: atest FrameworksServicesTests:com.android.server.pm.UserManagerTest#testAddTooManyUsers_Demo
Test: atest FrameworksServicesTests:com.android.server.pm.UserManagerTest
Test: atest FrameworksMockingServicesTests:com.android.server.pm.UserManagerServiceMockedTest
Change-Id: I117659792735c46c2982b4e895d6629f1a9c8fa4