[ios] Configure audio session once with AVAudioSessionModeVoicePrompt

The audio session is once configured during initialisation and then each time a
audio session is set active. This commit configures the audio session only once
when the app is started. In addition, it sets AVAudioSessionModeVoicePrompt,
which is recommended for apps that use text-to-speech.

Signed-off-by: Fabian Wüthrich <me@fabwu.ch>

iphone/Maps/Core/TextToSpeech/MWMTextToSpeech.mm

@@ -101,10 +101,11 @@ using Observers = NSHashTable<Observer>;
 
     NSError * err = nil;
     if (![[AVAudioSession sharedInstance] setCategory:AVAudioSessionCategoryPlayback
-                                          withOptions:AVAudioSessionCategoryOptionMixWithOthers |
-                                                      AVAudioSessionCategoryOptionDuckOthers
+                                          mode:AVAudioSessionModeVoicePrompt
+                                          options:AVAudioSessionCategoryOptionInterruptSpokenAudioAndMixWithOthers |
+                                                  AVAudioSessionCategoryOptionDuckOthers
                                                 error:&err]) {
-      LOG(LWARNING, ("[ setCategory]] error.", [err localizedDescription]));
+      LOG(LWARNING, ("Couldn't configure audio session: ", [err localizedDescription]));
     }
 
     // Set initial StreetNamesTTS setting
@@ -240,18 +241,11 @@ using Observers = NSHashTable<Observer>;
     stopSession();
     return;
   } else {
-    AVAudioSessionMode mode = AVAudioSessionModeDefault;
-    if ([MWMCarPlayService shared].isCarplayActivated) {
-      mode = AVAudioSessionModeVoicePrompt;
-    }
-    if (![[AVAudioSession sharedInstance] setCategory:AVAudioSessionCategoryPlayback
-                                                 mode:mode
-                                              options:
-                                                  AVAudioSessionCategoryOptionInterruptSpokenAudioAndMixWithOthers |
-                                                  AVAudioSessionCategoryOptionDuckOthers
-                                                error:nil] ||
-        ![[AVAudioSession sharedInstance] setActive:YES error:nil])
+    NSError * err = nil;
+    if (![[AVAudioSession sharedInstance] setActive:YES error:&err]) {
+      LOG(LWARNING, ("Couldn't active audio session: ", [err localizedDescription]));
       return;
+    }
 
     for (NSString * notification in turnNotifications)
       [self speakOneString:notification];