[ExoPlayer] Using MergingDataSource

I'm using AndroidVideoCache v2.7.0 and ExoPlayer v2.4.1, I have no trouble playing & caching a **single** media, however, I can't find a way to cache two medias (audio & video) at the same time.

To get into the details, I'm retrieving the URLs of the audio and video medias separately, I tried to cache both medias at the same time but it would then throw an IllegalStateException when passing the URI to ExoPlayer. Removing the cache functionality, passing both URI to ExoPlayer with MergingMediaSource works well and no errors are thrown.

So i'm starting to guess AndroidVideoCache cannot cache two or more medias at the same time. I dived a bit into the code and noticed the library open a single socket bound to the local address per instance. Even if I tried to create 2 instances of HttpProxyCacheServer, it doesn't work aswell.

首先使用 AndroidVideoCache v2.7.0 和 ExoPlayer v2.4.1 重现双媒体场景，通过 MergingMediaSource 使用单独的音频和视频 URL。检查 HttpProxyCacheServer 以及报告的 IllegalStateException，然后确定是否支持同时缓存；当行为得到修复或其限制得到明确确认时，即视为完成。

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