Move Recents task to back when hiding view.

Whenever the Recents activity actively hides itself move its task
to the back of the activity stack behind the home task. Otherwise
we get into a loop of the bck key returning to the Recents task
when it is intended to move to the home app.

Fixes bug 9750207.

Change-Id: I7d69f28368db148cb493bf54867df084907910ec