Add support for dozing after screen off.

On some devices, we want the screen off transition to complete before
we start dozing.  Added a new config.xml attribute config_dozeAfterScreenOff
to configure this behavior.

Defer starting dreams until the display is ready.

Fixed some minor issues in the system UI doze service when setting the
display state.

Bug: 16187655
Change-Id: Ib1bc60de5457166f4b4880732db5df989dda67a4