Allow battery voltage to be -1

The default value for voltage in BatteryStatsImpl was -1, so it was
implicitly "supported" and we should be prepared to store and
retrieve this value in BatteryHistory

Bug: 321187655
Test: atest --host PowerStatsTestsRavenwood:BatteryStatsHistoryIteratorTest
Change-Id: I2bf098a56857debdea74dd3ff60d12f9dbb90744