Adding support for using system loopback as AEC reference behind a flag

Run with
--enable-features=SystemLoopbackAsAecReference:added_delay_ms/100

This CL introduces SystemLoopbackListener. This class is an
implementation of DeviceOutputListener, which is used to provide a
reference signal to the AudioProcessor for chrome-wide echo
cancellation.

Whenever SystemLoopbackListener has at least one listener, it opens a
system loopback input stream with a callback that forwards the system
audio to all the listeners.

There is one SystemLoopbackListener in the whole AudioService, and it is
owned by StreamFactory, similar to OutputDeviceMixerManager (which is
the existing implementation of DeviceOutputListener, which provides a
mix of all output streams for an audio device).

This is intended for testing, so the CL leaves out error handling and
proper logging for now. We also log the stream as an InputController
so that it will show up in media-internals, this should be changed
to its own category later.

Bug: 412581642
Change-Id: I991c971701b1a0cd840326903ac6a9887a01194d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6432456
Auto-Submit: Fredrik Hernqvist <fhernqvist@google.com>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Fredrik Hernqvist <fhernqvist@google.com>
Reviewed-by: Olga Sharonova <olka@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1451852}