Use ExecutorService intead of ThreadPoolExecutor in AsyncHttpClient

Changed the setter to use ExecutorService because that's its threadPool
member type and Executors.newFixedThreadPool() returns an ExecutorService
(this method is referenced from the setThreadPoll() javadoc)

Author: erikghonyan

library/src/main/java/com/loopj/android/http/AsyncHttpClient.java

@@ -81,7 +81,6 @@
 import java.util.WeakHashMap;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
-import java.util.concurrent.ThreadPoolExecutor;
 import java.util.zip.GZIPInputStream;
 
 
@@ -331,10 +330,10 @@ public void setCookieStore(CookieStore cookieStore) {
      * Overrides the threadpool implementation used when queuing/pooling requests. By default,
      * Executors.newFixedThreadPool() is used.
      *
-     * @param threadPool an instance of {@link ThreadPoolExecutor} to use for queuing/pooling
+     * @param threadPool an instance of {@link ExecutorService} to use for queuing/pooling
      *                   requests.
      */
-    public void setThreadPool(ThreadPoolExecutor threadPool) {
+    public void setThreadPool(ExecutorService threadPool) {
         this.threadPool = threadPool;
     }