Do not delete stacks that are not empty.

The possibility existed that an activity was set to a task that it was
already being set to. If that were to happen, and it was the only
activity in the only task of the stack the stack would be deleted.
This fixes that situation and logs it as well to confirm that it does
fix bug 11272935. Logging to be deleted upon successful monkey run
exhibiting the log.

Change-Id: I436fdcc9a3734adad81d3ef90f29b93b3ac4dfcd