adds iOS audio interruption handling and recovery

src/common/ios.h

@@ -78,6 +78,12 @@ bool setAudioMixWithOthers(bool mixEnabled);
  **/
 bool hasBackgroundMusic();
 
+/**
+ * Registers notifications to handle and restore audio interruptions
+ **/
+void initAudioSessionInterruptionHandler();
+void destroyAudioSessionInterruptionHandler();
+    
 /**
  * Gets the area in the window that is safe for UI to render to (not covered by
  * the status bar, notch, etc.)