elm_interface_scrollable: change return value for post events.

elm_interface_scrollable: change return value for post events.

Summary:
1. Scroller in scroller case, callback process should continue.
     If child scroller does not scroll, parent scroller should be checked whether scroll or not.
     So returning EINA_TRUE is necessary.
2. down.dragged flag is always EINA_FALSE, so scroll is never unlocked.
@fix

Test Plan:
precondition
  elementary_config -> scrolling -> Set Scroll Smooth Amount to 0
elementary_test -> pagescroller

Reviewers: woohyun, zmike, singh.amitesh, cedric

Reviewed By: cedric

Subscribers: #committers, cedric, zmike

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D6187

Reviewed-by: Cedric BAIL <cedric@osg.samsung.com>