Avoid duplication of AppWindowToken in created Task

When WindowManagerService creates a new Task, it passes atoken to the
Task constructor. In this case atoken is added to mAppTokens list by
the Task constructor and then it is added manually again by calling
newTask.mAppTokens.add(atoken). As a result, the same atoken is present
in mAppTokens list twice.

When another window token is added to the list, it may be placed in
beetwen duplicated tokens and corresponding window will be hidden from
the screen by corresponding duplicated windows.
From user perspective it means that some windows will not be displayed
on the screen.

The issue is fixed by avoid adding atoken to mAppTokens list manually
when it was added already in Task constructor.

Change-Id: I10628e68186160fffdde07beb7d84ab3cecb7051