Change defaults to iOS defaults (#7)

Author: hiroshihorie

sdk/objc/components/audio/RTCAudioSessionConfiguration.m

@@ -69,11 +69,11 @@ - (instancetype)init {
     // By default, using this category implies that our app’s audio is
     // nonmixable, hence activating the session will interrupt any other
     // audio sessions which are also nonmixable.
-    _category = AVAudioSessionCategoryPlayAndRecord;
-    _categoryOptions = AVAudioSessionCategoryOptionAllowBluetooth;
+    _category = AVAudioSessionCategorySoloAmbient;
+    _categoryOptions = 0;
 
     // Specify mode for two-way voice communication (e.g. VoIP).
-    _mode = AVAudioSessionModeVoiceChat;
+    _mode = AVAudioSessionModeDefault;
 
     // Set the session's sample rate or the hardware sample rate.
     // It is essential that we use the same sample rate as stream format