No need to waste time downloading all this when it's readily available :)
Also use the official action to setup Java 8.
Also build both architectures (armv7 and arm64v8) and generate the APK,
so we can upload it.
Remove now unused and outdated `misc/ci/android-tools-linux.sh`.
(cherry picked from commit 5f19e1d571)
Removes AppVeyor and all Travis jobs but the iOS one, which hasn't been ported
to GitHub Actions yet (should be done soon).
Backports new style scripts from `master` branch too to do the same checks.