get_apk.py: Fetch builds from the release directory first

[JohanLorenzo]

For non-nightly builds, we fetch builds from the candidate directory https://archive.mozilla.org/pub/mobile/candidates/. This was useful when get_apk.py was used to publish non released builds, but that can be harmful if someone wants to download Firefox 60.0.

Let's change the default behavior to download APKs from https://ftp.mozilla.org/pub/mobile/releases/ instead