Implement downloading artifacts from Android builds.

BUG=chromium:512668
TEST=local run
download credential file from Cloud console
start devserver:
./devserver.py --static_dir=/usr/local/google/home/dshi/images \
  --log=/tmp/devserver.log --port 8082
  -a "/usr/local/google2/chromiumos/src/platform/dev/android_build_cred.json"
test download:
success:
curl http://127.0.0.1:8082/stage?target=shamu-userdebug\&build_id=2040953\&artifacts=test_zip\&branch=git_mnc-release
fail:
curl http://127.0.0.1:8082/stage?target=shamu-userdebug\&build_id=2040953\&artifacts=test_zip\&branch=none

cros flash a test device

Change-Id: I967d80ed3013f6085ae6d7e56e34b0f9dd9af10e
Reviewed-on: https://chromium-review.googlesource.com/304664
Commit-Ready: Dan Shi <dshi@chromium.org>
Tested-by: Dan Shi <dshi@chromium.org>
Reviewed-by: Simran Basi <sbasi@chromium.org>
5 files changed