Add method to calculate last full charge time

Bug: 38481300
Test: RunSettingsRoboTests
Change-Id: Icd551921f263352eb01d0c6ada62d277c8699507

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

@@ -261,6 +261,19 @@ public class BatteryUtils {
         });
     }
 
+    /**
+     * Calculate the time since last full charge, including the device off time
+     *
+     * @param batteryStatsHelper utility class that contains the data
+     * @param currentTimeMs      current wall time
+     * @return time in millis
+     */
+    public long calculateLastFullChargeTime(BatteryStatsHelper batteryStatsHelper,
+            long currentTimeMs) {
+        return currentTimeMs - batteryStatsHelper.getStats().getStartClockTime();
+
+    }
+
     private long convertUsToMs(long timeUs) {
         return timeUs / 1000;
     }