Pause all music when the app enters background

[PatriceJiang]

invoke AudioEngine::pauseAll when the app enters background

ref https://github.com/cocos-creator/2d-tasks/issues/1744

[PPpro]

这里是不是应该只暂停正在播放的音乐呢？ @PatriceJiang
可以看下这个 issue：https://github.com/cocos-creator/2d-tasks/issues/1886

Android 的 AudioEngine 实现好像有一部分重复处理了，这里是不是应该去掉呢

void AudioEngineImpl::onEnterBackground(const CustomEvent& event)
{
    // _audioPlayerProvider->pause() pauses AudioMixer and PcmAudioService,
    // but UrlAudioPlayers could not be paused.
    if (_audioPlayerProvider != nullptr)
    {
        _audioPlayerProvider->pause();
    }

    // pause UrlAudioPlayers which are playing.
    for (auto&& e : _audioPlayers)
    {
        auto player = e.second;
        if (dynamic_cast<UrlAudioPlayer*>(player) != nullptr
            && player->getState() == IAudioPlayer::State::PLAYING)
        {
            _urlAudioPlayersNeedResume.emplace(e.first, player);
            player->pause();
        }
    }
}

void AudioEngineImpl::onEnterForeground(const CustomEvent& event)
{
    // _audioPlayerProvider->resume() resumes AudioMixer and PcmAudioService,
    // but UrlAudioPlayers could not be resumed.
    if (_audioPlayerProvider != nullptr)
    {
        _audioPlayerProvider->resume();
    }

    // resume UrlAudioPlayers
    for (auto&& iter : _urlAudioPlayersNeedResume)
    {
        iter.second->resume();
    }
    _urlAudioPlayersNeedResume.clear();
}

[PatriceJiang]

是的，参考android 的实现，应该保存被暂停的id在 enter foreground 的时候 resume。
需要加一个接口，返回被暂停的id。