Revert "update_engine: Deprecate major version 1"

This partially reverts commit 55c75417e22d5026971276997924a345d9973bbc.

It turns out that we forgot a scenario when we deprecated major version
1. We use update_engine in lab tests (specifically
autoupdate_EndToEndTests on stable channel) to update a DUT to an
old (very old) versions using actual update payloads so we can test that
they can get updated to newer versions. However, deprecating major
version 1 in the update_engine caused trouble because we no longer can
update from a newer version to a version before M72 (to prepare the
device for update test). We need to put this feature back until we find
a better solution for it.

On this CL, we only support major version 1 in the client and only for
test (non-official) images. We don't even bother adding paygen support
for it.

This CL should be reverted once we figured out what to do with
provisioning the autoupdate end to end tests.

BUG=chromium:1043428
TEST=FEATURES=test emerge-reef update_engine
TEST=cros deployed it, then cros flash using an m71 payload, it succeeded.

Change-Id: I1fecbe3ae845b2e419f0999adc53e4732b1f7696
Reviewed-on: https://chromium-review.googlesource.com/c/aosp/platform/system/update_engine/+/2013884
Reviewed-by: Tianjie Xu <xunchang@google.com>
Reviewed-by: Sen Jiang <senj@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Commit-Queue: Amin Hassani <ahassani@chromium.org>

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