Fix division by zero in maybeAdjustDesiredStockLevelLocked().

If getBatteryScreenOffDischargeMah() returns zero that will lead to
a division by zero when computing estimatedLifeHours that will
bring down the entire system server. Guard against this by returning
from the function early if it returns zero.

Change-Id: I4905d7184e5185a07e4185fa0e263559d85cff49