BatteryUsageStats atom - frameworks/base

Writes the BatteryUsageStats atoms.proto atoms
based on the current BatteryUsageStats data in BatteryStats.

Does NOT write the past pre-reset snapshot atoms; that is
an adventure for a future cl.

Bug: 184095105
Test: atest BatteryUsageStatsProtoTests
Test: statsd_testdrive <atomId>
Change-Id: I2fc5a983deb58d7d393c0696db2165b124c94dc2