codePushNativeModule: launch async tasks in parallel (#693)

AsyncTasks are executed on a single thread to avoid common application errors caused by parallel execution.
So to make it possible to use tools like AsyncStorage while CodePush downloading an update we should use
`asyncTask.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);` code instead of `asyncTask.execute()`

Author: sergey-akhalkov

android/app/src/main/java/com/microsoft/codepush/react/CodePushNativeModule.java

@@ -290,7 +290,7 @@ public void dispatchDownloadProgressEvent() {
             }
         };
 
-        asyncTask.execute();
+        asyncTask.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
     }
 
     @ReactMethod
@@ -364,7 +364,7 @@ protected Void doInBackground(Void... params) {
             }
         };
 
-        asyncTask.execute();
+        asyncTask.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
     }
 
     @ReactMethod
@@ -416,7 +416,7 @@ protected Void doInBackground(Void... params) {
             }
         };
 
-        asyncTask.execute();
+        asyncTask.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
     }
 
     @ReactMethod
@@ -485,7 +485,7 @@ public void onHostDestroy() {
             }
         };
 
-        asyncTask.execute();
+        asyncTask.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
     }
 
     @ReactMethod