Use system loopback as AEC reference depending on settings flag

This makes the AudioService use the `use_loopback_aec_reference` flag in
AudioProcessingSettings to decide if chrome-wide AEC or loopback AEC
should be used. (this flag used to be `needs_loopback_aec_reference`,
this CL renames it to `use_loopback_aec_reference`.

This changes the role of the SystemLoopbackAsAecReference flag. Instead
of forcing the use of loopback AEC (which was the previous behaviour),
it now only decides if LoopbackReferenceManager should be created.

To still allow us to test loopback AEC, we now add an option to force
the use of loopback AEC to the command line. Specifying
`--enable-features=SystemLoopbackAsAecReference:forced_on/true` now uses
loopback AEC whenever chrome-wide AEC would be used.

Note that this CL is intended to be landed after
https://chromium-review.googlesource.com/c/chromium/src/+/6670033

Bug: 415954529

Change-Id: I6dfcf4f8692e8d311998433c5e51ae819618a5cb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6668134
Reviewed-by: Colin Blundell <blundell@chromium.org>
Commit-Queue: Fredrik Hernqvist <fhernqvist@google.com>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Olga Sharonova <olka@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1479730}