Android按返回鍵退出程序但不銷毀，程序后台保留

重寫onKeyDown事件即可

@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
    if (keyCode == KeyEvent.KEYCODE_BACK) {
        moveTaskToBack(false);
        return true;
    }
    return super.onKeyDown(keyCode, event);
}