【Android back键退出操作（不销毁当前activity退出app操作）】只需在activity页面加上如下代码即可：
@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
if(keyCode == KeyEvent.KEYCODE_BACK){
moveTaskToBack(true);
return true;
}
return super.onKeyDown(keyCode, event);
}
