elm_scroller: fix scrolling with key move

elm_scroller: fix scrolling with key move

Use content_region_show instead of content_pos_set in _key_action_move

Summary:
When user keep pressing key down or else on scroller content, scroller
animation is lagging because of elm_interface_scrollable_content_pos_set
by step_x or step_y value.  When focus moved to next object by press key
down or else, content_pos_set by ecore_animator continuously.  In this
time, content_pos_set in _key_action_move by step_x or step_y value
caused animation lagging problem.  I fixed to use content_region_show
instead of content_pos_set in _key_action_move for remove exist
animator.

Test Plan:
1. elementary_test -> Scroller3
2. Press 3 times "Append 10 Items in 3s" button
3. focus to Item1 and keep pressing key_down

Reviewers: jpeg, woohyun

Subscribers: cedric, jpeg

Differential Revision: https://phab.enlightenment.org/D5278

Change-Id: Ia112e52594ad1189fe767bcc7bb0c37a82bcc3e6
Signed-off-by: Jeonghyun Yun <jh0506.yun@samsung.com>