Use AT channel when determining build type

Similar to chromecasts, we now check the release channel to determine
the device build type.

If the AT build is a user build, the returned build type
will be BUILD_PROD if the channel is (1) empty or (2) stable-channel.
Otherwise, the returned build type is BUILD_BETA.

dogfood-channel, ensure GetBuildType returns BUILD_BETA.

Bug: None
Test: Build internal, flash on user build, set channel to
Change-Id: I02a629943fc3d7e91b103bb54b5e6d88595b7849
Reviewed-on: https://chromium-review.googlesource.com/999100Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Commit-Queue: Aidan Wolter <awolter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548855}

chromecast/base/cast_sys_info_android.cc

@@ -54,9 +54,9 @@ CastSysInfo::BuildType CastSysInfoAndroid::GetBuildType() {
   if (!base::StringToInt(CAST_BUILD_INCREMENTAL, &build_number))
     build_number = 0;
 
-  // Note: no way to determine which channel was used on play store.
+  const std::string channel(GetSystemReleaseChannel());
   if (strcmp(build_info_->build_type(), kBuildTypeUser) == 0 &&
-      build_number > 0) {
+      build_number > 0 && (channel.empty() || channel == "stable-channel")) {
     return BUILD_PRODUCTION;
   }