Generate OTA packages for A/B update.

It calls brillo_update_payload to generate the payload for A/B update.
And packages the payload according to Android OTA package format.

Note that it only supports generating full/incremental OTAs with this
CL. Signing for release may not work properly at the moment.

Bug: 25715402
Change-Id: I4ac8505bacad28a572a9320dc8b52dd0f1ce47f5