BatteryUsageStats - use bus end time

Now that BUS contains an end time and a duration, use
those in the atom instead of estimating them from
the clock time that the request was made.

Bug: 187223764
Test: atest BatteryUsageStatsProtoTests
Change-Id: Ie69d42bfd19c3a0055e96a25ac33c16c0f7d1a3f