Squid: Avoid cycle between java packages

https://trello.com/c/A0rrIWb0

Resolved packages dependency cycle in MOTECH Mobile Forms API
* Moved FormGroupValidator to validator
* Moved validators implementations to validator.impl
* Moved Study to domain

Change-Id: Icef1db796f3ae5359f9ffdaf9e0ee4d0bd53b22a
Merge "Squid: Avoid cycle between java packages"