[Chromecast] Add product type for Android Things

Also shifts the is_android_things GN arg to upstream, so this will need
to be landed at the same time as removing it in internal.

BUG=internal b/37788362
TEST=Verified new product type applies correctly on Android Things
builds.

Change-Id: Ie524dc2a930b1ebb13337f97d4103e1914033780
Reviewed-on: https://chromium-review.googlesource.com/532836
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Bryan Henry <bryanhenry@google.com>
Cr-Commit-Position: refs/heads/master@{#479179}

• chromecast/base/BUILD.gn[diff]
• chromecast/chromecast.gni[diff]
• components/metrics/proto/cast_logs.proto[diff]