bugfix: use correct AudioSource for Android Marshmallow and above

On Android Marshmallow (6) and above, no recordings were possible and
no files would be created. Instead, we get an error message regarding
audio capture.

[This
issue](#41),
however, briefly describes the solution, which is basically this
patch.

Now the calls seems to be recorded correclty (tested on Android
6.0.1).

Note that even though the source says just `MIC`, both voices in the
call are captured.

Author: kristianlm

src/com/callrecorder/android/RecordService.java

@@ -198,7 +198,11 @@ private void startRecording(Intent intent) {
 		recorder = new MediaRecorder();
 
 		try {
-			recorder.setAudioSource(MediaRecorder.AudioSource.VOICE_CALL);
+			int source = MediaRecorder.AudioSource.VOICE_CALL;
+			// For Marshmallow and above, VOICE_CALL doesn't work but MIC does:
+			// https://developer.android.com/reference/android/os/Build.VERSION_CODES.html#M
+			if(android.os.Build.VERSION.SDK_INT >= 23) source = MediaRecorder.AudioSource.MIC;
+			recorder.setAudioSource(source);
 			recorder.setOutputFormat(MediaRecorder.OutputFormat.THREE_GPP);
 			recorder.setAudioEncoder(MediaRecorder.AudioEncoder.AMR_NB);
 			fileName = FileHelper.getFilename(phoneNumber);