Currently, we sign the APKs that were already signed with the QA key.
However, this makes it impossible to copy our signature on top of APKs
built independently, which would be the last step to reproduce our
builds.

With this commit, we provide a patch to go back to the unsigned (but
already aligned) APK, so that we can keep shipping QA-signed APKs for
testers, but we can go back to the unsigned APK for the final signing.

The changes to apply this patch in the signing scripts will be part of
another commit.