Prevent activity being destroyed immediately if embedded

The embedded activity was destroyed immediately when being
finished because the next top activity was on the adjacent
TaskFragment and was already visible. Therefore, the finishing
animation was played before the organizer requested to
finish the activity on the adjacent TaskFragment.

Prevent the last activity of the embedded TaskFragment to
be removed immediately if the organizer requested to.

Bug: 189386466
Test: finish both activities on separate TaskFragments

Change-Id: I916eddc4dcf0de3bc7ed7264296f441d1b7bb726