- Seperates coreaudio implementations for iOS and OS X.
- Manages input/ouput port overrides when layer is reset via calls
  to Dring::setAudioOutputDevice in response to an audio route override
  event listened for by the iOS client.
- TODO: use AVAudioSession API with graphs in Objective-C++, or
  optimally, in Swift in the iOS/OS X clients.

Change-Id: I6ba34a8bdce7bedd3b9e9ca9bbcd2cc8caccf3f4