Previously `ChannelMixingAudioProcessor` output float because it was
implemented using the audio mixer's float mixing support.
Move the implementation over to just using the `ChannelMixingMatrix` and make
it publicly visible in the common module so it can be used by apps for both
playback and export.
Also resolve a TODO that no longer had a bug attached by implementing support
for putting multiple mixing matrices to handle different input audio channel
counts, and fix some nits in the test code.
Tested via unit tests and manually configuring a `ChannelMixingAudioProcessor`
in the transformer demo app and playing an audio stream that identifies
channels, and verifying that they are remapped as expected.
PiperOrigin-RevId: 523653901
affc237055