e_policy_visibility: fix bug for checking a visible state of transient_child

e_policy_visibility: fix bug for checking a visible state of transient_child

There was a bug that the parent window was uniconified even though its child was not visible.
This was because that the child was uniconic state but its visibility was unknown state.
(this means it was not calculated visibility and it was not shown yet).

To resolve this problem, we add code to check whether child's visibility is unknown
in _e_policy_check_transient_child_visible function.

Change-Id: I80604558afa86fac020e0681a3bcdb8adb75bb84