Resume audio on iOS after phone call or alarm

When a phone call or an alarm triggers on iOS, the application receives
an "audio interruption" and it's up to the application to resume
playback when the interruption ends. I added handling for audio
interruptions same as if the game is focused out and then back in.

drivers/coreaudio/audio_driver_coreaudio.h

@@ -90,6 +90,7 @@ public:
 	virtual void finish();
 
 	bool try_lock();
+	void stop();
 
 	AudioDriverCoreAudio();
 	~AudioDriverCoreAudio();