UM.canAddMoreUsers(userType)

Introduces an API for whether more users can be added of the given user
type.

Bug: 192577100
Bug: 175795666
Bug: 189937255
Bug: 205100630
Test: atest com.android.server.pm.UserManagerTest#testAddTooManyUsers
Change-Id: I1c705d65d1c083a60fd4e058456dcdc4ffa0fffb