The current signing method uses apksigner to sign the JAR so that it
will automatically select algorithms that are compatible with Android
SDK 23, which added the most recent algorithms:
https://developer.android.com/reference/java/security/Signature

This signing method uses then inherits the default signing algothim
settings, since Java and Android both maintain those.  That helps
avoid a repeat of being stuck on an old signing algorithm.  That means
specifically that this call to apksigner does not specify any of the
algorithms.

The old indexes must be signed by SHA1withRSA otherwise they will no
longer be compatible with old Androids.

apksigner 30.0.0+ is available in Debian/bullseye, Debian/buster-backports,
Ubuntu 21.10, and Ubuntu 20.04 from the fdroid PPA.  Here's a quick way to
test:

for f in `ls -1 /opt/android-sdk/build-tools/*/apksigner | sort ` /usr/bin/apksigner; do printf "$f : "; $f sign --v4-signing-enabled false; done

closes #1005