ui.box_flow: refactor layout_update

Summary:
This patch remove evas_box function from Efl.Ui.Box_Flow and add unit test.
Depends on D8214

Test Plan:

1. make check
2. elementary_test -to 'efl.ui.box' with 'flow' checkbox.

Reviewers: zmike, Jaehyun_Cho

Reviewed By: zmike

Subscribers: cedric, reviewers, committers

Tags: efl

Differential Revision: https://phab.enlightenment.org/D8219