Squid: Avoid cycle between java packages

https://trello.com/c/A0rrIWb0

Resolved packages dependency cycles in MOTECH Scheduletracking API
* Extracted interface of EnrollmentService
* fetch() and filter() methods of Criteria interface doesn't depend on
  EnrollmentService anymore
* Moved some logic from EnrollmentService to Enrollment class
* Rewrote the Criteria tests

Change-Id: Ibe9c2cce51008dcb8725dd338f9870ff2cefcba1