TODO: Investigate AVAudioSession.setCategory defaults

[ryanheise]

In an effort to allow the Dart API to pass in any combination of parameters with default values, the plugin chooses AVAudioSessionCategoryOptions.none as the default for the options parameter. However, the documentation says that allowBluetoothA2db will effectively be switched on by default "if you configure an app’s audio session to use the AVAudioSessionCategoryAmbient, AVAudioSessionCategorySoloAmbient, or AVAudioSessionCategoryPlayback categories". I may need to either emulate iOS's default values or try to avoid setting the default value myself and let iOS choose the default.