Improve screen brightness boost behavior.

Wake-up when entering brightness boost mode, don't boost in ambient
mode since some display device drivers do strange things in that mode and
boost doesn't work.  Waking up feels more natural as well.

Don't flutter the power HAL's interactive mode bit simply due to changes
in display ready state since that may result in visible artifacts
such as display flashes.

Don't stop the auto-brightness sensor while temporarily boosted.

Don't prevent the display from entering the ready state while in brightness
boost since that would unnecessarily delay the transition from DOZING to AWAKE
until boost is finished.

Restart the user activity timeout when brightness boost ends and prevent
the display from dimming while boosted.

The pixel fairies basked in the sunlight.

Bug: 18262044
Bug: 18261782
Change-Id: I8c42a1e6091b0fe1253e90265ac248087ebc24e1