Exclude inactive UIDs from PowerStats aggregation

This is an optimization that skips processing UIDs that have
not recorded any PowerStats - effectively skips adding zeros.

Bug: 395954049
Test: atest PowerStatsTests; atest PowerStatsTests:BatteryUsageStatsProviderPerfTest#getBatteryUsageStats_accumulated
Flag: EXEMPT bugfix

Change-Id: I76dce3f78e60dfc3d7dd1c7c453256c010718028