Improving library stability next to other audio application

-Set the audio buffer size programmatically in SPeechRecognizer
-BUGFIX : error when the microphone is still used by another application. (fix the "Could not get audio input for record source" error)
-BUGFIX : check that the AudioRecord object is well initialized before startRecording (fix the "IllegalStateException: startRecording() called on an uninitialized AudioRecord" exception)