atspi: fix infinite loop issue

atspi: fix infinite loop issue

It is possilbe to get a target object as a return value of get_object_at_point.
In this case _elm_widget_elm_interface_atspi_component_accessible_at_point_get
works again with the target which is same with root object.

[Enhanced]
- We do not have to go parent if the bottom up logic meets root object.
- Visiblity of panel is always true. So we are checking its SHOWING state.
  If the panel is not SHOWING, remove it from valid children list.

Change-Id: I2edeefdc9a0c5955f1c28926321b74424c71bc40