Add idle-state controller for the Task Manager

"Idle time" is currently defined to begin ~ an hour (actually slightly
more) after the screen turns off and/or dreaming begins.  Screen-on
and dreams-ending are the triggers for declaring "idle" time to be
over.

Bug 14993295

Change-Id: I0898871d5b76a52d647ae2ebcb1b2f941ec45e79