Hide deprecated UserManager.getUsers(boolean, boolean, boolean)

It was added to include pre-created users (3rd parameter), which is
deprecated; this CL makes it private, and future CLs will completely
remove it.

Test: m -j update-api # it was a TestingApi
Test: build and flash
Flag: EXEMPT cleaning up deprecated code
Bug: 253528462

Change-Id: Id0cba5f4873740db050e0a524e1ed7efb2016f10