audiomixerbin: Add skelleton class

Change-Id: I78854b5da441b38ee04a0b7b88b09294d839d4bd
audiomixerbin: Add ALWAYS srcpad

Change-Id: I645bb883ede3d214c97ce5ea874b6aa47d1ca8f6
audiomixerbin: Implement requested pad

Change-Id: I9da0b6bbca7713ecc8902076d719726c9d6561c7