This makes apksigner a hard requirement of the signing procedure.
We'll first try to find a globally installed version from PATH and if
that's not available fall back to using a version from build-tools.
Future TODO: always sign with apksigner, blocked on signature transplant
support for apksigv2/v3
Closesfdroid/fdroidserver#634Closesfdroid/fdroidserver#827
Marcus Hoffmann