e_service_launcher: fixed wrong operation for the backward direction

e_service_launcher: fixed wrong operation for the backward direction

activity: callee ec
given ec: callee ec (transient_for child)  <-- destroying by back key
          caller ec (transient_for parent) <-- will be shown

In this case, the given ec is a callee ec and is transient_for with caller ec (lc) by
group launching. Also, because of transient_for, caller ec is not iconified.
Since the given ec is destroyed by the back key, we can't find out caller ec(lc) with activity and given ec.
We have to use transient_for to find caller ec(lc).

Change-Id: I82e1fa23000c78739866b4ead570d740c088815d