batteryservice: support shutdown at low battery voltage

Add logic to shutdown when the battery continues discharging at
SOC=0 with a charger plugged-in. The shutdown is initiated when
the battery voltage falls below a cutoff threshold which can
be configured by the android property 'ro.cutoff_voltage_mv'.

CRs-Fixed: 1011948
Change-Id: Iabb317ac1a7aedf28d6edbb3cd88c64ad4706534