fix: add keepAliveTime to close idle threads

Author: jonathanedey

src/main/java/com/google/firebase/internal/FirebaseThreadManagers.java

@@ -23,7 +23,10 @@
 import java.util.Set;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
+import java.util.concurrent.LinkedBlockingQueue;
 import java.util.concurrent.ThreadFactory;
+import java.util.concurrent.ThreadPoolExecutor;
+import java.util.concurrent.TimeUnit;
 
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -85,7 +88,10 @@ protected ExecutorService doInit() {
       ThreadFactory threadFactory = FirebaseScheduledExecutor.getThreadFactoryWithName(
           getThreadFactory(), "firebase-default-%d");
 
-      return Executors.newFixedThreadPool(100, threadFactory);
+      ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(100, 100, 60L,
+          TimeUnit.SECONDS, new LinkedBlockingQueue<Runnable>(), threadFactory);
+      threadPoolExecutor.allowCoreThreadTimeOut(true);
+      return threadPoolExecutor;
     }
 
     @Override