[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}

• chromecast/BUILD.gn[diff]
• chromecast/base/BUILD.gn[diff]
• chromecast/base/cast_sys_info_android.cc[diff]
• chromecast/browser/BUILD.gn[diff]
• chromecast/browser/android/BUILD.gn[diff]
• chromecast/browser/android/apk/src/org/chromium/chromecast/shell/CastSysInfoAndroidThings.java[Added - diff]
• chromecast/chromecast.gni[diff]