elm_bg: restore legacy bg sizing_eval to keep backward compatibility

elm_bg: restore legacy bg sizing_eval to keep backward compatibility

In tizen 4.0, elm_bg size is always (0, 0). only internal image object had size
as file image size.

To keep legacy size policy, elm_bg override `elm_layout_sizing_eval` to
resize internal image and `efl_canvas_group_calculate` to prevent resize elm_bg.

Note that if you don't set any size to elm_bg, elm_bg size is (0, 0) by default
even though its child have size.

@tizen_fix

Change-Id: I54677a1024078ba1f157b8fd495e00dce713cce6