Export BatteryStats events to Atrace.

Similar to http://ag/19779936, this reports event details to Perfetto.
All events are recorded as instants. Ideally, the start/finish would
correspond to a single async segment, but that requires storing unique
cookie ids for every event (if not managed carefully, can grow in
memory). Instead, we can generate synthetic segments in Perfetto itself.

Bug: 245749764
Test: atest FrameworksServicesTests:BatteryStatsTests
Change-Id: If24c0d52c76a9cba6692ae5de20b80cb759b9a4d