pins the barcode scanner aar used in the android build and its 2
dependencies to a sha256 hash using a script `fetch_barcode_scanner.sh`
which is called in the process of building the apk by `make_apk.sh`.
It fetches the 3 aar files if not already existing, puts them in
`/contrib/android/aars` and verifies their shasum against the hardcoded
hashes in `fetch_barcode_scanner.sh`.
