Add BatteryUsageStats.getDischargeDurationMs

This method returns the total amount of time the battery was
discharging during the current battery session.

Also, fixing time units used in BatteryStats tests: they are
supposed to be microseconds, not milliseconds.

Bug: 214407774
Test: atest FrameworksCoreTests:BatteryChargeCalculatorTest
Change-Id: I5b64a6bd8a577b558b90cb60a1db8c3774121921