Added UserFilter and new methods on UMS that use it.

Currently, UMS has many getUsers() methods, which can take up to
4 arguments (like getUsersInternal(boolean excludePartial,
boolean excludeDying, boolean excludePreCreated,
boolean resolveNullNames)); this approach is already hard to use
(for example, it pollutes the code with call-site comments like
/* excludePartial= */ true) and maintain (for example, to add other
conditions like onlyFullAdmins).

So, this CL adds a new UserFilter POJO, and 2 getUsers() methods that
uses it, althoguh it doesn't refactor the existing methods to use
it yet (as they're not unit-tested). It also adds a getNumberOfUsers()
method which will be used to count the number of admins in some
scenarios.

Flag: EXEMPT new code that's not used yet
Test: atest FrameworksMockingServicesTests:UserFilterTest
Test: atest -it FrameworksMockingServicesTests --test-filter=.*testGetUsersWithFilterAndGetNumberOfUsers.*
Bug: 419086491

Change-Id: Id0cba5f4873740db050e0a524e1ed7efb2016f12