Add feature flag for battery settings page.

Create PowerUsageSummaryLegacy and power_usage_summary_legacy, which
contains the old implementaion.

Bug: 69797586
Test: RunSettingsRoboTests
Change-Id: Icb9c25f06889462a6eedea48ae166043a6636848

src/com/android/settings/fuelgauge/PowerUsageSummary.java

@@ -251,7 +251,7 @@ public class PowerUsageSummary extends PowerUsageBase implements
 
     @Override
     public int getMetricsCategory() {
-        return MetricsEvent.FUELGAUGE_POWER_USAGE_SUMMARY;
+        return MetricsEvent.FUELGAUGE_POWER_USAGE_SUMMARY_V2;
     }
 
     @Override