[Chromecast] Add loopback input for AEC

Add i2s loopback in order to enable the reference channel for AEC.

Bug: http://b/38428792
Test: built cast_shell_internal_apk, ran assistant with AEC
Change-Id: Ie8c1ecf09bc31050e3cfe61eddf69dc21aba1fce
Reviewed-on: https://chromium-review.googlesource.com/566063
Commit-Queue: Nat Jeffries <njeff@google.com>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488801}

• chromecast/BUILD.gn[diff]
• chromecast/base/chromecast_switches.cc[diff]
• chromecast/base/chromecast_switches.h[diff]
• chromecast/chromecast.gni[diff]
• chromecast/media/cma/backend/android/BUILD.gn[diff]
• chromecast/media/cma/backend/android/DEPS[diff]
• chromecast/media/cma/backend/android/cast_media_android.cc[diff]
• chromecast/media/cma/backend/android/loopback_audio_manager.cc[Added - diff]
• chromecast/media/cma/backend/android/loopback_audio_manager.h[Added - diff]