Merge "releasetools: Support generating downgrade A/B OTAs." into nyc-dev am: 5a4508bcef am: e7847765a6 am: a7dcbfb2d8 am: 00f2147e70 am: aaabaf74d6  -s ours
am: 3ebe86b6a8  -s ours

Change-Id: I3abc63909aeb5d011153f5c2e418851582b926a9