Bug Fix: Deadlock in AVAudioSession

Problem: Stopping the ADTClient when the app goes into the background and starting it when the app is resumed
         causes a deadlock.

Background: We were running the AVAudioSession record in a separate thread. It held a lock on the app's audio session.
            When the application is resumed it tries to reinitialize the audio session and waits for the lock.

Solution: Run the AVAudioRecorder from the main thread.