Fix transition animation when going back twice

The logic in ag/5108931 was intended to suppress the next return
animation if it would cross a task boundary, while preserving return
animations in all other cases.  However, it accidentally caused
excessive suppressions in the case of repeated back button presses.  The
reason is that although the sending activity is indeed only topOfTask
when moving forward cross-task, it's *always* topOfTask when pressing
back, so the state of the sending activity doesn't contain sufficient
information to feed this logic.

This patch changes to suppress when the receiving activity is task root,
which is a more direct and reliable signal.  (By definition, the return
transition from an activity will cross a task boundary if and only if
it's the task root.)

Fixes: 177780499
Test: New test in ag/13593522

Change-Id: Ia5751da6122dc1ddb557f45318712185fa0b3df9