make synchornized

Author: VinayGuthal

firebase-common/src/main/java/com/google/firebase/heartbeatinfo/DefaultHeartBeatController.java

@@ -151,7 +151,7 @@ private DefaultHeartBeatController(
 
   @Override
   @NonNull
-  public HeartBeat getHeartBeatCode(@NonNull String heartBeatTag) {
+  public synchronized HeartBeat getHeartBeatCode(@NonNull String heartBeatTag) {
     long presentTime = System.currentTimeMillis();
     HeartBeatInfoStorage storage = storageProvider.get();
     boolean shouldSendGlobalHB = storage.shouldSendGlobalHeartBeat(presentTime);