BatteryStatsImpl:fix ArithmeticException due to list size is zero.

Test: OEM monkey test
Bug: 341658103
Change-Id: Idfc3db1c0ea3be536cad82bf98fab6d668d26a36

services/core/java/com/android/server/power/stats/BatteryStatsImpl.java

@@ -3377,7 +3377,7 @@ public class BatteryStatsImpl extends BatteryStats {
             }
             return mTotalTimeUs + (mNesting > 0
                     ? (curBatteryRealtimeUs - mUpdateTimeUs)
-                            / (mTimerPool != null ? mTimerPool.size() : 1)
+                            / (mTimerPool != null && mTimerPool.size() > 0 ? mTimerPool.size() : 1)
                     : 0);
         }