UserTypeFactory configurable setMaxAllowed

We provide support in config_user_types.xml to change the max number of
users allowed of a given user type. This could, e.g., allow multiple
Guest users. Note that even though it can be configured, that doesn't
mean it'll work - to have more than one Guest user additional work in
UMS and other places would be required.

Test: atest UserManagerServiceUserTypeTest
Bug: 256690588
Change-Id: I02b212bb9a046bbf5662fea05d833f1717e23327