Ensure consistency of BatteryUsageStats deltas

To maintain a consistent state of accumulated battery usage stats
we need to make sure that every update provides the same breakdown
(process state, battery state, screen state) as the persistent
instance.

Bug: 374882485
Test: atest PowerStatsTests
Flag: EXEMPT bugfix
Change-Id: I844e440171ce34869a69c0ce7c7d6db0cdb3a335