Prevent concurrent modification of battery history in tests

Bug: 270159981
Test: atest FrameworksServicesTests:BatteryStatsTests
Change-Id: I8a031a64a1d7d0f3e9c3abafdae6cd722f639408

services/tests/servicestests/src/com/android/server/power/stats/MockBatteryStatsImpl.java

@@ -231,6 +231,10 @@ public class MockBatteryStatsImpl extends BatteryStatsImpl {
         setExternalStatsSyncLocked(mExternalStatsSync);
     }
 
+    @Override
+    public void writeSyncLocked() {
+    }
+
     public static class DummyExternalStatsSync implements ExternalStatsSync {
         public int flags = 0;