If task isn't found in recents, look in the stacks.

It may take an attach to move a task into the recents list. If the
timing is right a task may not be in recents by the time we are
removing it from the stacks due to a finish. In that case we should
look in the stacks for the task as well as looking in recents.

Fixes bug 18017409.

Change-Id: Idcfe2e263c9d0fe9a063fdf22515ac4e7fe89ecb