AudioMixer: Preserve stereo volume in multi-channel mixing

CrOS exposes 4-channel output for ARC++ and mixes samples in CRAS
server to real channel format, but AudioMixer will discard right
channel volume when mixing a 2-channel track to 4-channel output.
This behavior will cause ARC++ fail on Audio Frequency Line Test
in CTS-V since the right channel can't be tested.

The code is also updated to work for 3, 5, 6, 7, 8 canonical output
channel position masks, as well as continue the mono volume
handling for output channel index masks.

Bug: 110551766
Test: Run CTS-V Audio Frequency Line Test
Test: atest mixerops_benchmark
Change-Id: I4c6ee86d30bb8296f0e32f9a17b1135e1313fd64