[Contextual Edu] Move business logic to ContextualEducationInteractor

Change ContextualEducationRepository from class to interface because it has a duplicate role as ContextualEducationInteractor.
All business logic would be in ContextualEducationInteractor instead.

Test: KeyboardTouchpadEduInteractorTest
Test: ContextualEducationRepositoryTest
Bug: 317496783
Flag: com.android.systemui.keyboard_touchpad_contextual_education

Change-Id: Icac43b1e00957da3be9bdc7b4f7da9d4a5a533bd