Attemp to fix an NPE while transferStartingWindow.

The NPE scenario may happen when:
Start Task with activity(A), then start anoterh task with activity(B),
so there should be an app transition happen to Open B and Close A,
which will start the exit animation on A.
When Activity(A) receive onAnimationFinished, it will remove all
windows with onExitAnimationDone, thus remove the starting window on A.
So if there is another activity(C) below A and it will set to visible,
there will try to transfer the starting window to C.
We shouldn't allow that from happen because window is in the middle of
remove process.

Bug: 201019560
Test: presubmit
Change-Id: I5880896252ea3207d67753552aefa3f1c9f354b3