Effect for multichannel PCM downmix to stereo

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