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* Adds new step to decrypt configuration seed which comes from DM Server.
* Adds configuration selection UI.

BUG=chromium:829361
TEST=TBD

Cq-Include-Trybots: luci.chromium.try:closure_compilation
Change-Id: I299cdf7b8994fd929a7f15303bfb21ebf78996ba
Reviewed-on: https://chromium-review.googlesource.com/1090714
Commit-Queue: Roman Sorokin <rsorokin@chromium.org>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Reviewed-by: Lutz Justen <ljusten@chromium.org>
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568879}