Add a method to calculate battery usage diff results from history

Bug: 184807417
Test: make SettingsRoboTests
Test: make SettingsGoogleRoboTests
Change-Id: If613c5cfd7e00e1883107385d3d8552774378cd7

tests/robotests/src/com/android/settings/fuelgauge/BatteryHistEntryTest.java

@@ -161,5 +161,7 @@ public final class BatteryHistEntryTest {
             .isEqualTo(BatteryManager.BATTERY_STATUS_FULL);
         assertThat(entry.mBatteryHealth)
             .isEqualTo(BatteryManager.BATTERY_HEALTH_COLD);
+        assertThat(entry.getKey())
+            .isEqualTo("com.google.android.settings.battery-" + entry.mUserId);
     }
 }