AudioFlinger: Use optimized code for stereo downmix

Test: atest channelmix_benchmark
Test: atest channelmix_tests
Test: atest downmix_benchmark
Test: atest downmix_tests
Bug: 230414312
Bug: 238397904
Merged-In: I51dd67cce01c498cc0557c8dd4a0a21bc94a7c6a
Change-Id: I51dd67cce01c498cc0557c8dd4a0a21bc94a7c6a
4 files changed