Add support for audio device selection in AppRTCDemo.

Summary:

- Creates a list of available (possible to select) audio devices.
- Automatically selects (routes audio) the "best/default" audio device.
- If possible, starts a proximity sensor that will switch between headset earpiece and speaker phone based on how close the a person's ear the mobile device is held.

TBR=glaznev

BUG=4103,4109

Review URL: https://webrtc-codereview.appspot.com/31239004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7978 4adac7df-926f-26a2-2b94-8c16560cd09d