Fix transition return issues when more than 1 level depth

d85bed51, Adding support for cross-task hero transition, When running
cross-task, only play the enter animation, but the judgment for start
exit transition with shared element is broken. For the case on the issue,
when the exit transition completes, the activity is on the top of task,
it is not allowed to execute return transition.

This CL modifies the condition to allow the return transition. For start
exit back transition, the isReturning is always true. We can use it to
decide if it is allowed to play the return transition.

Bug: 137838129
Test: atest ActivityTransitionTest
      Manual testing on ApiDemos with Activity transition

Change-Id: Iaa6a875dbe305f6356887616b797dc45e76a4b56