Reduce screen on latency, eliminate flashes.

Always use the ElectronBeam now, even when we are only animating
the backlight so that we will have a black surface remaining
on the screen after the screen turns off.

When turning on the screen, keep the black surface showing until
we unblock screen on then dismiss it as usual.

This change eliminates the flashing of old display content when
the screen is turned on.  It also helps to conceal some of the
latency of turning the screen on.  We always turn the screen on
immediately (even when screen on has nominally been blocked) and
rely on the black surface to hide the screen contents until the
last moment.  Dismissing the black surface is practically
instantaneous compared to turning the screen on.

Bug: 7299370
Bug: 7139924
Change-Id: I57d13287acd05bd0a48811095bb02dc7bc7cbeb6