Floss: define MMC interface

In this patch, we define common codec methods (init, cleanup, transcode)
in |MMCInterface|, a wrapper that routes data to the appropriate
receiver or implementation. The caller of codec libraries would use the
interface to access basic codec utilities.

Bug: 285999597
Tag: #floss
Test: m - None
Change-Id: I8f6489d60b3afcc276035509a4633ad31b10e1fc