Fix Bluetooth audio in calls.

// FREEBIE

Signal/src/call/CallAudioService.swift

@@ -98,14 +98,15 @@ import AVFoundation
         if call.hasLocalVideo {
             setAudioSession(category: AVAudioSessionCategoryPlayAndRecord,
                             mode: AVAudioSessionModeVideoChat,
-                            options: .defaultToSpeaker)
+                            options: [.defaultToSpeaker])
         } else if call.isSpeakerphoneEnabled {
             setAudioSession(category: AVAudioSessionCategoryPlayAndRecord,
                             mode: AVAudioSessionModeVoiceChat,
-                            options: .defaultToSpeaker)
+                            options: [.defaultToSpeaker])
         } else {
             setAudioSession(category: AVAudioSessionCategoryPlayAndRecord,
-                            mode: AVAudioSessionModeVoiceChat)
+                            mode: AVAudioSessionModeVoiceChat,
+                            options: [.allowBluetooth])
         }
     }
 
@@ -150,7 +151,7 @@ import AVFoundation
         } else {
             setAudioSession(category: AVAudioSessionCategoryPlayAndRecord,
                             mode: AVAudioSessionModeVoiceChat,
-                            options: .mixWithOthers)
+                            options: [.mixWithOthers, .allowBluetooth])
         }
 
         // HACK: Without this async, dialing sound only plays once. I don't really understand why. Does the audioSession
@@ -299,6 +300,8 @@ import AVFoundation
     private func setAudioSession(category: String,
                                  mode: String? = nil,
                                  options: AVAudioSessionCategoryOptions = AVAudioSessionCategoryOptions(rawValue: 0)) {
+        assert(Thread.isMainThread)
+
         do {
             if #available(iOS 10.0, *), let mode = mode {
                 try AVAudioSession.sharedInstance().setCategory(category, mode: mode, options: options)