Fast mixer configuration

Add compile-time option for when to use fast mixer.

Double HAL frame count for fast tracks due to SRC, and make the normal
frame count multiplier an even number for compatibility.  Sample rate
conversion can result in underruns if the HAL frame count is used as is,
due to jitter.

Change-Id: Ia1f8da1b8ac247d9807acfce3c318161db000905