audio_streams: add num_channels in ShmStream trait

Add num_channels() to allow to get the channel count of a stream.

BUG=b:157433024
TEST=cargo build

Change-Id: If52e6531e0f2e76603163697203e7fd857f676ec
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/adhd/+/2259312
Reviewed-by: Chih-Yang Hsia <paulhsia@chromium.org>
Auto-Submit: Judy Hsiao <judyhsiao@chromium.org>
Tested-by: Judy Hsiao <judyhsiao@chromium.org>
Commit-Queue: Judy Hsiao <judyhsiao@chromium.org>
2 files changed