More paths for turning on screen immediately.

This fix forces the path through the updateLightsLocked method to turn
the screen on immediately if mWaitingForFirstLightSensor is true. Also
do not clear mWaitingForFirstLightSensor if mPreparingForScreenOn
is true. Wait until it turns false.

Fixes bug 6612418.

Change-Id: I03407e748cce4906a73de1f15df1654649b133c4