Use UM.canAddMoreUsers and UM.isUserTypeEnabled

Settings shouldn't allow creating users if the type is diabled, and it
shouldn't allow creating more of a particular user type if no more can
be created (even if more can be created of other user types).
Previously, Settings didn't take into account the user type when
querying whether more users could be created.

Bug: 192577100
Test: com.android.settings.users.UserSettingsTest
Change-Id: I065478fd14779f528be4edce8ae215391a752ef4