navigation_layout: set efl.content y-axis align to 0.0

Summary:
If `efl.content` min size is greater than edc area size, `efl.bar` and
`efl.content` are overlapped because default y-align is 0.5.

This patch avoid it.

Test Plan:
Sample code:
```
// gcc -o efl_ui_stack_example efl_ui_stack_example.c `pkg-config --cflags --libs elementary`

EAPI_MAIN void
efl_main(void *data EINA_UNUSED, const Efl_Event *ev EINA_UNUSED)
{
   Eo *win = efl_add(EFL_UI_WIN_CLASS, efl_main_loop_get());

   Eo *stack = efl_add(EFL_UI_STACK_CLASS, win);
   efl_content_set(win, stack);

   Eo *nl = efl_add(EFL_UI_NAVIGATION_LAYOUT_CLASS, stack,
                    efl_ui_stack_push(stack, efl_added));

   Eo *bn = efl_add(EFL_UI_NAVIGATION_BAR_CLASS, stack);
   efl_text_set(bn, "Title Text");
   efl_ui_navigation_layout_bar_set(nl, bn);

   Eo *btn = efl_add(EFL_UI_BUTTON_CLASS, nl,
                     efl_text_set(efl_added, "Button 1"),
                     efl_gfx_hint_size_min_set(efl_added, EINA_SIZE2D(50, 80)),
                     efl_content_set(nl, efl_added));

   efl_gfx_entity_size_set(win, EINA_SIZE2D(100, 80));
}
EFL_MAIN()
```

Reviewers: Jaehyun_Cho

Reviewed By: Jaehyun_Cho

Subscribers: cedric, #reviewers, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D8868

data/elementary/themes/edc/efl/navigation_layout.edc

@@ -21,6 +21,7 @@ group { "efl/navigation_layout";
                relative: 0.0 1.0;
             }
             rel2.to_x: "base";
+            align: 0.5 0.0;
          }
       }
    }