Some APK need to be included in the final images.

The APKs could be built from within the Android build system. However doing that
as-is would have the side effect of having them signed with the distribution
keys.

In turn this could prevent the users that are using F-Droid (or other package
managers using the F-Droid repositories) from being able to upgrade these
applications.

In addition using the Android distribution keys could make things harder as it
would be up to the distribution to handle the migration from one key set to
another.

Including the F-Droid APKs as prebuilt fixes all that.

Alternatively we could also try to build them in a reproducible way and embed
F-Droid's signature inside the final APK, but that probably requires more work.