First pass at implementing an audio effect whose role is to
downmix multichannel PCM buffers to stereo.
The effect is not handling volume changes.
The effect code here handles quad, 4.0, 5.1 and 7.1 input configurations,
to optimize the most commom configurations, and does not yet handle
generic multichanel configurations.
Change-Id: I74d04bd961348f3f0e4ae7714b70e620808a0829
64143abe1f