ios/macos: use non-exclusive audio session

Using this option means the OS  won't pause other
sources of audio when playback starts in MPV.

This matches the behaviour on other platforms.

audio/out/ao_audiounit.m

@@ -115,7 +115,9 @@ static bool init_audiounit(struct ao *ao)
 
     MP_VERBOSE(ao, "max channels: %ld, requested: %d\n", maxChannels, (int)ao->channels.num);
 
-    [instance setCategory:AVAudioSessionCategoryPlayback error:nil];
+    [instance setCategory:AVAudioSessionCategoryPlayback
+              withOptions:AVAudioSessionCategoryOptionMixWithOthers
+                    error:nil];
     [instance setMode:AVAudioSessionModeMoviePlayback error:nil];
     [instance setActive:YES error:nil];
     [instance setPreferredOutputNumberOfChannels:prefChannels error:nil];