Modify task navigation to return to recent tasks.

Tasks launched from the recent task list will now return to the list
when they are finished. Also tasks that are launched from the
notification panel and services will now return to the list,
provided that the launcher is not front and center when they are
launched.

Fixes bug 14464114.

Change-Id: Ic0d3731fc7248d1eaa80e5ee399753d80e80c979