GeckoView is built in two steps (build to create the arch-specific
.aars and build_apk to build the Kotlin code).
However, they share big chunks of build code, therefore we always risk
to forget to update one of the two.
So, in this commit we move the duplicate parts in a shared preamble
file.

After we updated our toolchains for Firefox 128, we encountered
reproducibility problems in the license files.
Therefore, we build a patched plugin to resolve them.