Max users only applies to switchable users

* config_multiuserMaximumUsers now only applies to non-Guest non-Demo
  switchable users.
* In particular, profiles are decoupled from config_multiuserMaximumUsers
  * Therefore, profiles and secondary users do not "use up" each other's
    slots:
    * Profiles can be created even if secondary users is at max.
    * Whether or not another secondary user can be created is independent
      of whether there are profiles on the device.
  * Private profiles are no longer implicitly disabled just because
    switchable users is disabled; they would need to be disabled
    explicitly on their own.
* Guests also don't count towards or consider the secondary user limit
  (which was also the case prior to this year). Same for Demo users.
* Configuration of each user type max must be finite; we no longer allow
  unlimited since we can no longer rely on an overall max capping it.

This cl updates the core logic in UserManager. Pending work not in this
cl includes updating Tradefed, adb shell commands, and tests.

Flag: android.multiuser.decouple_max_users_from_profiles
Bug: 394178333
Bug: 407111025
Bug: 428754296
Test: atest FrameworksServicesTests:com.android.server.pm.UserManagerTest

Change-Id: I6561ff3d662234cd4b965dbb01af23160e4b4199