android退出程序 后台运行 实现退出键和home键一样的效果

 public boolean onKeyDown(int keyCode, KeyEvent event) {
         PackageManager pm = getPackageManager();
         ResolveInfo homeInfo =
             pm.resolveActivity(new Intent(Intent.ACTION_MAIN).addCategory(Intent.CATEGORY_HOME), 0);
         if (keyCode == KeyEvent.KEYCODE_BACK) {
             ActivityInfo ai = homeInfo.activityInfo;
             Intent startIntent = new Intent(Intent.ACTION_MAIN);
             startIntent.addCategory(Intent.CATEGORY_LAUNCHER);
             startIntent.setComponent(new ComponentName(ai.packageName, ai.name));
             startActivitySafely(startIntent);
             return true;
         } else
             return super.onKeyDown(keyCode, event);
     }
     private void startActivitySafely(Intent intent) {
         intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
         try {
             startActivity(intent);
         } catch (ActivityNotFoundException e) {
             Toast.makeText(this, "null",
                     Toast.LENGTH_SHORT).show();
         } catch (SecurityException e) {
             Toast.makeText(this, "null",
                     Toast.LENGTH_SHORT).show();
         }
     }