[Chromecast] Return release channel from AT UpdateEngine

Android Things UpdateEngine has an API for retrieving the current update
channel. We now return this to sys_info as the release channel.

1. Separate Android Things sys_info from Android
2. Return correct release channel from AT UpdateEngine

BUG=72137029
TEST=Build, Check returned release channel

Change-Id: If60b698eb833c54cd4899b647a91b6073c60e071
Reviewed-on: https://chromium-review.googlesource.com/996916
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Commit-Queue: Aidan Wolter <awolter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549227}

diff --git a/chromecast/chromecast.gni b/chromecast/chromecast.gni
index 93e1eca3..1907fb6 100644
--- a/chromecast/chromecast.gni
+++ b/chromecast/chromecast.gni

@@ -84,6 +84,8 @@
 declare_args() {
   enable_volume_tables_access =
       is_android_things && chromecast_branding != "public"
+  is_android_things_non_public =
+      is_android_things && chromecast_branding != "public"
 }
 
 declare_args() {