Allow dreams to wake up gently.

This change adds a new DreamService.wakeUp() method to allow a
dream to wake itself up gently and perform a transition before
finally finishing.  The power manager will ask a dream to wake
up gently in most cases but may force it to happen immediately
when necessary.

If the dream takes too long to finish itself then the dream
controller will douse it with water summarily after 5 seconds.

Change-Id: Ib0564c5650cd5a454e1acc5aa91fe46995eecfa7