Use the update-binary in the source build for downgrade OTAs.

We used to use the update-binary from the target build when creating
incremental OTAs. But for downgrade OTAs, we should use the one in the
source build instead, which is actually newer.

Bug: 27556903
Change-Id: Ib6415729b979dbffdebdda24902f7f560942801a
1 file changed