[CodeHealth] Rename channel_span() to channel()

Replace uses of `AudioBus::channel_span()` with the identical
`AudioBus::channel()`.

These changes are pure renames + reformatting, there are no functional
changes.

Directories covered in this split:
chromeos

This CL was uploaded by an experimental version of git cl split
(https://crbug.com/389069356).

Bug: 373960632
Change-Id: Ib577d13cb9ee597fc4718034620c7ba03cf71364
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7254422
Reviewed-by: James Cook <jamescook@chromium.org>
Auto-Submit: Thomas Guilbert <tguilbert@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1558266}