LE Audio stereo recording

With this patch, we can receive ISO data for two channels. If audio
framework expect to receive mono signal, we would mix it into mono stream.
If audio framework expects stereo signal, we will pass it through.

Test: make a mono phone call, make stereo phone call, drop connection
during phone call, ensure no crashes and good audio quality
Bug: 209004829
Bug: 150670922
Change-Id: Ie8c347d15b08006360597c8493711a283eaef18c