EffectDownmix: Move processing to audio_utils ChannelMix

Separate out the downmix processing into an audio_utils class
ChannelMix.  The new downmix processing uses a general matrix.

Test: downmix_benchmark
Test: downmix_tests
Bug: 188502620
Bug: 189336830
Change-Id: I43eb2bbe17471c8c9a5a4ce3e65918001ea975ee