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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}