More control over audio attributes / audio category

[andreasgangso]

Android and ios allows signalling what kind of media the app is playing, in order to better understand when it should automatically stop playback etc. Right now we are always aggressively signalling in a way that causes other apps to stop playing as soon as you open apps with bccm_player. E.g. because we are setting AVAudioSessionCategoryPlayback.

Investigate and improve this / make it configurable.