Add one backend DAI, and add one mixer control to route playback stream to the new backend DAI. Start/stop playback commands will be sent to CVD if starting/stopping playback, after voice call is established. Change-Id: I334f1894ac0800e6a71fa7e20ac634b1b05cc414 Signed-off-by: Helen Zeng <xiaoyunz@codeaurora.org>