switch to using apksigner by default, as much as possible
Since @Bubu implemented !736 (merged), the buildserver setup has signed about 1430 APKs with apksigner, so we know it works well. This merge request flips the whole APK signing over to apksigner, removing support for using jarsigner in fdroid publish. The Android ecosystem expects apksigner to make APK signatures now. Also, apksigner is already installed on the production boxes, and a good version of it has been included in build-tools since version 26.0.2.
Other parts will still use jarsigner, like index signing, since that's a JAR anyway, and APK verification in fdroid update, where jarsigner is only a fallback option. Ultimately, that should be replaced by pure Python #94
should fix admin#202 (comment 533571535)
Edited by Hans-Christoph Steiner