EffectDownmix: Update for new ChannelMix matrix

Downmix matrix compliant with the following standards:
ITU-R 775-2, ATSC A/52, ETSI TS 101 154, IEC 14496-3,
which is unity gain for the front left and front right channel
contribution.

Correspondingly, output amplitude is allowed to be +3dBFS
from 0dBFS previously.

Test: atest downmix_benchmark
Test: atest downmix_tests
Bug: 188502620
Bug: 189336830
Change-Id: I4c06a2ea0e4619d8a5607fbeb68ee65b663aaf55