PowerManagerService: fix getAutoBrightnessValue for btn/kbd

getAutoBrightnessValue calculations are correct only for LCD
backlight brightness values increasing with higher light values.
Skip the calculation for the reversed button/keyboard backlight
arrays, where the backlight brightness decreases with increasing
light values.

Change-Id: Ib72b8325fe2b0f83780e91dc94eea2e38061224b