Implemented accessors for derived submix HAL objects.

This change cleans up casting from contained to container structures.
Such that if..

struct child {
  struct parent;
  int child_data;
};

is changed to...

struct child {
  int child_data;
  struct parent;
};

functions that cast from "parent" to "child" will continue to function
correctly.

Change-Id: I88d70930d4d4df2e291239e562971b07d8b92f94
1 file changed