ios: use AVAudioSessionCategoryPlayback to support play in background

we must use playback to support background playing.
make sure you have enabled 'audio' tag ( or 'voip' tag ) at XCode -> Capabilities -> BackgroundMode

note:

this does NOT mean you can start/activate an audio session from background.

it only means iOS will not interrupt already activated playing audio
session when app move to background.

for start audio from background, we must use local notification

Author: zxcpoiu

ios/RNInCallManager/RNInCallManager.swift

@@ -372,9 +372,12 @@ class RNInCallManager: NSObject, AVAudioPlayerDelegate {
             self.mRingtone.numberOfLoops = -1 // you need to stop it explicitly
             self.mRingtone.prepareToPlay()
 
-            //self.audioSessionSetCategory(AVAudioSessionCategorySoloAmbient, [.DefaultToSpeaker, .AllowBluetooth], #function)
-            self.audioSessionSetCategory(AVAudioSessionCategorySoloAmbient, nil, #function)
+            // --- we must use playback to support background playing.
+            // --- make sure you have enabled 'audio' tag ( or 'voip' tag ) at XCode -> Capabilities -> BackgroundMode
+            //self.audioSessionSetCategory(AVAudioSessionCategoryPlayback, [.DuckOthers], #function)
+            self.audioSessionSetCategory(AVAudioSessionCategoryPlayback, nil, #function)
             self.audioSessionSetMode(AVAudioSessionModeDefault, #function)
+            //self.audioSessionSetActive(true, nil, #function)
             self.mRingtone.play()
         } catch let err {
             NSLog("RNInCallManager.startRingtone(): caught error=\(err)")