Fixes to ensure keyboard backlight is lit when the keyboard is visible.

- Ignore mKeyboardVisible in PowerManagerService::applyKeyboardState if
  KEYBOARD_BRIGHT_BIT is already set in mPowerState.  Previously, if the
  keyboard were to hide after setting mPowerState, but before running
  applyKeyboardState, then the keyboard backlight would remain off at the
  next slide event since mPowerState is unchanged.

- Always set mLightSensorKeyboardBrightness in
  PowerManagerService::lightSensorChangedLocked to a sensor-based value,
  instead of 0 when the keyboard is hidden.  This ensures
  mLightSensorKeyboardBrightness is sensible on slide events since it may
  not be recomputed timely.