官方指南: https://developer.android.google.cn/guide/topics/media-apps/index.html
官方Audio样例: https://github.com/googlesamples/android-UniversalMusicPlayer
AudioDemo是UniversalMusicPlayer的简化版
Media App architectures:
- audio apps: a client/server design
- video apps: a single-activity design