Fix doze jank by removing a fullscreen layer of overdraw

Move all doze related stuff into DozeScrimController, and combine
both alpha values from the regular ScrimController and
DozeScrimController before applying it to a view. Move the black
background from NotificationPanelView to DozeScrimController, which
saves a fullscreen layer of overdraw during the transition.

Bug: 18238168
Change-Id: Ifb133bf4a0f8255f5c2f5e205509af339cac8c8f