compositemixer: Add handle port function

Change-Id: I475083584b9a9fbf2450079419adb32e22043421
compositemixer: Add unhandle port function

Change-Id: I30a9536863e736d0295b2a431ca22d8627bc8cd7
audiomixer: Move constants to header file

Change-Id: Ifffb8e712532ab2b158d29f5b6f00ee0986ee581