修复快速切换，暂停、恢复功能时，概率性出现的重新下载问题，添加onPre()回调，onPre()用于请求地址之前执行界面UI更新操作

app/src/main/java/com/arialyy/simple/download/SingleTaskActivity.java

@@ -24,6 +24,7 @@ import android.os.Environment;
 import android.os.Handler;
 import android.os.Message;
 import android.support.v7.widget.Toolbar;
+import android.util.Log;
 import android.view.View;
 import android.widget.Button;
 import android.widget.RadioGroup;
@@ -243,6 +244,11 @@ public class SingleTaskActivity extends BaseActivity<ActivitySingleBinding> {
 
   private class MySchedulerListener extends Aria.DownloadSchedulerListener {
 
+    @Override public void onPre(String url) {
+      super.onPre(url);
+      Log.d(TAG, "url ==> " + url);
+    }
+
     @Override public void onNoSupportBreakPoint(DownloadTask task) {
       super.onNoSupportBreakPoint(task);
       T.showShort(SingleTaskActivity.this, "该下载链接不支持断点");