Fix error in MeasuredEnergySnapshot voltage

The voltage algorithm didn't seem quite right; e.g.
if avgVoltageMV == 1, this would give the wrong value.
We presumably should round off, not up. Indeed, the tests
in MeasuredEnergySnapshotTest already do so.

Bug: 173765509
Test: atest BatteryStatsTests
Change-Id: Iefc5011d2df01b2f24dd13145d39fdc4de34753f