[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}

diff --git a/chromecast/chromecast.gni b/chromecast/chromecast.gni
index 8426bea..4310f91 100644
--- a/chromecast/chromecast.gni
+++ b/chromecast/chromecast.gni

@@ -50,6 +50,12 @@
 }
 
 declare_args() {
+  # Currently android things libraries live in internal.  TODO(njeff): change
+  # this when Android Things API is moved to public
+  enable_athings_loopback = is_android_things && chromecast_branding != "public"
+}
+
+declare_args() {
   # Use Playready CDMs for internal non-desktop builds.
   use_playready = !is_cast_desktop_build && chromecast_branding != "public"
 }