AudioManager enhancement

[diegohaz]

• AudioManager must be the only person who knows what is playing. External classes must call playing(SongPost) or playing(Post), which return bool;
• External classes must be able to call current(SongPost) or current(Song), which differ from playing(), because return true even the song is paused;
• External classes must be able to call loading(SongPost) or loading(Song);
• External classes must be able to call play(SongPost) or play(Song);
• AudioManager should allow external classes to choose if they want whether play in background or not;
• AudioManager should release its current status (playing, loading, paused, stopped etc.);
• AudioManager could be able to identify currently playing song from external apps (like Apple Music and Spotify).

[diegohaz]

Use AVPlayer instead of AVAudioPlayer.