BatteryStatsHelper: Include screen on baseline power draw

Calculate the baseline power usage for the device when it is in suspend and idle.
The device is drawing POWER_CPU_IDLE power at its lowest power state.
The device is drawing POWER_CPU_IDLE + POWER_CPU_AWAKE power when a wakelock is held.
The device is drawing POWER_CPU_IDLE + POWER_CPU_AWAKE + POWER_SCREEN_ON when the screen is on.

Bug:27533512
Change-Id: Idcb587390bc8159fcbd6625cca4cb1aca19976d6