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.
Travis CI upgraded their Xenial VMs to default to openjdk11, which
is not working properly for sdkmanager, so we can no longer accept
licenses for the SDK.
They don't really seem to care about providing a good alternative
for us from the few threads I read, so let's just force openjdk8
back.
It has been failing since a few days, apparently `sdkmanager --licenses`
is no longer sufficient to accept all licenses, the `platform-tools` install
triggers another license dialog. Now saying yes to all just in case.
