Allow dreams to control screen state and brightness.

Added setDozeScreenState() and setDozeScreenBrightness() methods to
DreamService.  The values specified here only take effect once
startDozing is called and can be changed while dozing.

This required some significant rework of the display power controller
but the result seems quite nice and better represents the policy
we want to apply.

Changed the test dream a little bit to make it flash the screen
every minute using the new functions.

Bug: 15903322
Change-Id: I83bcc34503f1b87727d2b2b3c0ef08507f9f0808