The main goal of this CL is to remove old buffer handling using static arrays
and switch to the improved rtc::Buffer class instead.
By doing so, we can remove some members (since Buffer maintains them instead) and
do some additional cleanup.
This CL also fixes some minor style issues and improves the locking mechanism.
Finally, AudioDeviceBuffer::SetRecordingChannel() is deprecated since it has never been
used and is not included in any test.
BUG=NONE
Review-Url: https://codereview.webrtc.org/2333273002
Cr-Commit-Position: refs/heads/master@{#14661}