Improvements to power manager turning on screen.

The keyguard/window manager recently got a facility to report when it is
okay to turn the screen on, when it knows the lock screen is displayed.

The power manager was using this wrong, just using it to drive the
flags given to the input system.  Duh.

This change now uses the information to determine when to turn the screen
brightness up from 0.  For an OLED screen, this is the time when the
user can actually see anything on the screen.

For LCD screens this may not be optimal, because the LCD may start running
before its backlight is turned on, so if you look carefully you may see
stuff before it is lit up.  On the other hand, it is good to turn on the
display as early as possible (before waiting for the keyguard) because it
can take a little bit of time to get that and the touch screen going.  By
only waiting on the display brightness, we allow turning on the screen
in the kernel to proceed in parallel with ensuring the keyguard is displayed.

Change-Id: I7ee4ce19fd4efd5b51872b855af6263f53cd6c30