e_policy_stack: add code to check transient_for order

e_policy_stack: add code to check transient_for order

There was a bug that the transient_for children stack was incorrect.

1. Win_C1 set transient_for to Win_P already
2. Create Win_C2
3. Set Win_C2's transient_for to Win_P
4. Show Win_C2
5. Set Win_C1's transient_for to Win_P again

Above scenario, We exptected the window stack as below.
[Top]  Win_C1 -> Win_C2 -> Win_P  [Bottom]
But, window stack was Win_C2 -> Win_C1 -> Win_P.

This will fix this issue.

Change-Id: I1048bf5bbb492f52db5423ee610c2afe0e8e7097