Implement auto-sleep functionality.

Added a new SLEEP_TIMEOUT setting which governs how long the device will
remain awake or dreaming without user activity.  By default this
value is set to -1 which maintains today's existing behavior.

We basically represent the time we are allowed to be dreaming as a new
kind of user activity summary state called DREAM, similar to BRIGHT
and DIM.  When the sleep timeout expires, the state is cleared and
the dream ends.

Bug: 17665809
Change-Id: I59aa7648dcec215f1285464fc1134934a09230e5