Add payload prop to set custom proxy URL am: 98cb8f7aef

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2172130

Change-Id: I1999d3f0ec3a805dae1800dbd095f46d41007a0f
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>