Refactor anomaly detection

This cl refactor it so:
1. AnomayLoader could create its own BatteryStatsHelper if needed.
2. AnomalyDetector could detect the anomaly with a target package name
3. Remove FeatureProvider in AnomalyLoader and use
AnomalyDetectionPolicy instead

With this cl, we could add anomaly detection to battery usage page
more easily.

Bug: 36924669
Test: RunSettingsRoboTests
Change-Id: I1ee09701de8b5c8855f44e447813760285794c89