Play audio alert if child is disconnected

If the child device disconnects unexpectedly, alert the
user, in case they were not expecting the disconnect.

The audio file being played is originally from here:
https://freesound.org/people/pan14/sounds/263655/

README.md

@@ -36,3 +36,5 @@ proposed changed.
 
 App icon originals from [WPZOOM](http://www.wpzoom.com/wpzoom/new-freebie-wpzoom-developer-icon-set-154-free-icons)
 and formatted using [Android Asset Studio](https://romannurik.github.io/AndroidAssetStudio/index.html).
+
+Audio file originals from [freesound](https://freesound.org).

src/protect/babymonitor/ListenActivity.java

@@ -28,6 +28,7 @@ import android.app.Activity;
 import android.media.AudioFormat;
 import android.media.AudioManager;
 import android.media.AudioTrack;
+import android.media.MediaPlayer;
 import android.os.Bundle;
 import android.util.Log;
 import android.widget.TextView;
@@ -141,6 +142,11 @@ public class ListenActivity extends Activity
 
                 if(Thread.currentThread().isInterrupted() == false)
                 {
+                    // If this thread has not been interrupted, likely something
+                    // bad happened with the connection to the child device. Play
+                    // an alert to notify the user that the connection has been
+                    // interrupted.
+                    playAlert();
 
                     ListenActivity.this.runOnUiThread(new Runnable()
                     {
@@ -169,4 +175,26 @@ public class ListenActivity extends Activity
 
         super.onDestroy();
     }
+
+    private void playAlert()
+    {
+        final MediaPlayer mp = MediaPlayer.create(this, R.raw.upward_beep_chromatic_fifths);
+        if(mp != null)
+        {
+            Log.i(TAG, "Playing alert");
+            mp.setOnCompletionListener(new MediaPlayer.OnCompletionListener()
+            {
+                @Override
+                public void onCompletion(MediaPlayer mp)
+                {
+                    mp.release();
+                }
+            });
+            mp.start();
+        }
+        else
+        {
+            Log.e(TAG, "Failed to play alert");
+        }
+    }
 }