fake_omaha.py: Fix invalid full_update keyword

The correct key that clients and tests use is full_payload, not
full_update.

BUG=chromium:1157326
TEST=None

Change-Id: Ib0b1989f6a71d8d4c4d2c961f35ebbf248b24809
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/2593732
Commit-Queue: Amin Hassani <ahassani@chromium.org>
Commit-Queue: Kyle Shimabukuro <kyleshima@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Auto-Submit: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Sanika Kulkarni <sanikak@chromium.org>
Reviewed-by: Kyle Shimabukuro <kyleshima@chromium.org>
1 file changed