update_payload: Fix IsDelta to support payload v2.

Added check for the old_partition_info field which is used by major
version 2 payload.

Other changes are needed in order to pass presubmit lint check.

BUG=None
TEST=Run devserver with a v2 delta payload.

Change-Id: I3d7057fde44bd24d768177d5ed5c36bc2d4ca94b
Reviewed-on: https://chromium-review.googlesource.com/312952


Commit-Ready: Sen Jiang <senj@chromium.org>
Tested-by: Sen Jiang <senj@chromium.org>
Reviewed-by: Alex Deymo <deymo@chromium.org>
Reviewed-by: Gilad Arnold <garnold@chromium.org>